bcachefs: bcachefs_metadata_version_snapshot_deletion_v2

bcachefs: bcachefs_metadata_version_snapshot_deletion_v2

We're going to be speeding up snapshot deletion, by only having it
process the extents/dirents/xattrs btrees if an inode of a given
snapshot ID was present.

This raises the possibility of 'bkey_in_missing_snapshot' errors popping
up, if we ever accidentally don't do the corresponding inode update, or
if the new algorithm has bugs.

So instead of deleting snapshot IDs, add a new deleted flag, so that
'key in missing snapshot' errors can more definitively tell what
happened and automatically repair.

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