bcachefs: Add missing snapshots_seen_add_inorder()

This fixes an infinite loop when repairing "extent past end of inode",
when the extent is an older snapshot than the inode that needs repair.

Without the snaphsots_seen_add_inorder() we keep trying to delete the
same extent, even though it's no longer visible in the inode's snapshot.

Fixes: 63d6e931 ("bcachefs: bch2_fpunch_snapshot()")
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>