bcachefs: Print cycle on unrecoverable deadlock

bcachefs: Print cycle on unrecoverable deadlock

Some lock operations can't fail; a cycle of nofail locks is impossible
to recover from. So we want to get rid of these nofail locking
operations, but as this is tricky it'll be done incrementally.

If such a cycle happens, this patch prints out which codepaths are
involved so we know what to work on next.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>