bcachefs: Make check_key_has_snapshot safer

Snapshot deletion v2 added sentinal values for deleted snapshots, so
"key for deleted snapshot" - i.e. snapshot deletion missed something -
is safe to repair automatically.

But if we find a key for a missing snapshot we have no idea what
happened, and we shouldn't delete it unless we're very sure that
everything else is consistent.

So hook it up to the new bch2_require_recovery_pass(), we'll now only
delete if snapshots and subvolumes have recenlty been checked.

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