bcachefs fixes for 6.10-rc6

simple stuff:
- null ptr/err ptr deref fixes
- fix for getting wedged on shutdown after journal error

- fix missing recalc_capacity() call, capacity now changes correctly
  after a device goes read only

  however: our capacity calculation still doesn't take into account when
  we have mixed ro/rw devices and the ro devices have data on them,
  that's going to be a more involved fix to separate accounting for
  "capacity used on ro devices" and "capacity used on rw devices"

- boring syzbot stuff

slightly more involved:
- discard, invalidate workers are now per device
  this has the effect of simplifying how we take device refs in these
  paths, and the device ref cleanup fixes a longstanding race between
  the device removal path and the discard path

- fixes for how the debugfs code takes refs on btree_trans objects
  we have debugfs code that prints in use btree_trans objects. It uses
  closure_get() on trans->ref, which is mainly for the cycle detector,
  but the debugfs code was using it on a closure that may have hit 0,
  which is not allowed; for performance reasons we cannot avoid having
  not-in-use transactions on the global list.

  introduce some new primitives to fix this and make the synchronization
  here a whole lot saner
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