bcachefs: Runtime self healing for keys for deleted snapshots

bcachefs: Runtime self healing for keys for deleted snapshots

If snapshot deletion incorrectly missing some keys and leaves keys for
deleted snapshots, that causes a bit of a problem for data move - we
can't move an extent for a nonexistent snapshot, because the extent
might have to be fragmented, and maintaining correct visibility in child
snapshots doesn't work if it doesn't have a snapshot.

Previously we'd just skip these keys, but it turns out that causes
copygc to spin.

So we need runtime self healing, i.e. calling check_key_has_snapshot()
from the data move path.

Snapshot deletion v2 included sentinal values for deleted snapshot
nodes, so this is quite safe.

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