bcachefs: BCH_RECOVERY_PASS_NO_RATELIMIT

Add a superblock flag to temporarily disable ratelimiting for a recovery
pass.

This will be used to make check_key_has_snapshot safer: we don't want to
delete a key for a missing snapshot unless we know that the snapshots
and subvolumes btrees are consistent, i.e. check_snapshots and
check_subvols have run recently.

Changing those btrees - creating/deleting a subvolume or snapshot - will
set the "disable ratelimit" flag, i.e. ensuring that those passes run if
check_key_has_snapshot discovers an error.

We're only disabling ratelimiting in the snapshot/subvol delete paths,
we're not so concerned about the create paths.

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