bcachefs changes for 6.12-rc1

rcu_pending, btree key cache rework: this solves lock contenting in the
key cache, eliminating the biggest source of the srcu lock hold time
warnings, and drastically improving performance on some metadata heavy
workloads - on multithreaded creates we're now 3-4x faster than xfs.

We're now using an rhashtable instead of the system inode hash table;
this is another significant performance improvement on multithreaded
metadata workloads, eliminating more lock contention.

for_each_btree_key_in_subvolume_upto(): new helper for iterating over
keys within a specific subvolume, eliminating a lot of open coded
"subvolume_get_snapshot()" and also fixing another source of srcu lock
time warnings, by running each loop iteration in its own transaction (as
the existing for_each_btree_key() does).

More work on btree_trans locking asserts; we now assert that we don't
hold btree node locks when trans->locked is false, which is important
because we don't use lockdep for tracking individual btree node locks.

Some cleanups and improvements in the bset.c btree node lookup code,
from Alan.

Rework of btree node pinning, which we use in backpointers fsck. The old
hacky implementation, where the shrinker just skipped over nodes in the
pinned range, was causing OOMs; instead we now use another shrinker with
a much higher seeks number for pinned nodes.

Rebalance now uses BCH_WRITE_ONLY_SPECIFIED_DEVS; this fixes an issue
where rebalance would sometimes fall back to allocating from the full
filesystem, which is not what we want when it's trying to move data to a
specific target.

Use __GFP_ACCOUNT, GFP_RECLAIMABLE for btree node, key cache
allocations.

Idmap mounts are now supported - Hongbo.

Rename whiteouts are now supported - Hongbo.

Erasure coding can now handle devices being marked as failed, or
forcibly removed. We still need the evacuate path for erasure coding,
but it's getting very close to ready for people to start using.

Status, and when will we be taking off experimental:
----------------------------------------------------

Going by critical, user facing bugs getting found and fixed, we're
nearly there. There are a couple key items that need to be finished
before we can take off the experimental label:

- The end-user experience is still pretty painful when the root
  filesystem needs a fsck; we need some form of limited self healing so
  that necessary repair gets run automatically. Errors (by type) are
  recorded in the superblock, so what we need to do next is convert
  remaining inconsistent() errors to fsck() errors (so that all runtime
  inconsistencies are logged in the superblock), and we need to go
  through the list of fsck errors and classify them by which fsck passes
  are needed to repair them.

- We need comprehensive torture testing for all our repair paths, to
  shake out remaining bugs there. Thomas has been working on the tooling
  for this, so this is coming soonish.

Slightly less critical items:

- We need to improve the end-user experience for degraded mounts: right
  now, a degraded root filesystem means dropping to an initramfs shell
  or somehow inputting mount options manually (we don't want to allow
  degraded mounts without some form of user input, except on unattended
  servers) - we need the mount helper to prompt the user to allow
  mounting degraded, and make sure this works with systemd.

- Scalabiity: we have users running 100TB+ filesystems, and that's
  effectively the limit right now due to fsck times. We have some
  reworks in the pipeline to address this, we're aiming to make petabyte
  sized filesystems practical.
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