bcachefs fixes for 6.10-rc5

Lots of (mostly boring) fixes for syzbot bugs and rare(r) CI bugs.

The LRU_TIME_BITS fix was slightly more involved; we only have 48 bits
for the LRU position (we would prefer 64), so wraparound is possible for
the cached data LRUs on a filesystem that has done sufficient
(petabytes) reads; this is now handled.

One notable user reported bugfix, where we were forgetting to correctly
set the bucket data type, which should have been BCH_DATA_need_gc_gens
instead of BCH_DATA_free; this was causing us to go emergency read-only
on a filesystem that had seen heavy enough use to see bucket gen
wraparoud.

We're now starting to fix simple (safe) errors without requiring user
intervention - i.e. a small incremental step towards full self healing.
This is currently limited to just certain allocation information
counters, and the error is still logged in the superblock; see that
patch for more information. ("bcachefs: Fix safe errors by default").
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