bcachefs: BCH_SNAPSHOT_DELETED -> BCH_SNAPSHOT_WILL_DELETE

bcachefs: BCH_SNAPSHOT_DELETED -> BCH_SNAPSHOT_WILL_DELETE

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 we'll want to be able to differentiate more definitively between
'snapshot went missing' (and perhaps needs to be reconstructed), and
'key in snapshot that was deleted'.

So instead of deleting snapshot IDs, we'll be adding a new deleted flag
and leaving them permanently.

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