bcachefs: bch2_checksum() returns 0 for unknown checksum type

This fixes missing guards on trying to calculate a checksum with an
invalid/unknown checksum type; moving the guards up to e.g. btree_io.c
might be "more correct", but doesn't buy us anything - an unknown
checksum type will always be flagged as at least a checksum error so we
aren't losing any safety doing it this way and it makes it less likely
to accidentally pop an assert we don't want.

Reported-by:  <syzbot+e951ad5349f3a34a715a@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>