bcachefs: Check for subvolues with bogus snapshot/inode fields

This fixes an assertion pop in btree_iter.c that checks for forgetting
to pass a snapshot ID when iterating over snapshots btrees.

Reported-by:  <syzbot+0dfe05235e38653e2aee@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>