bcachefs: Fix deleted inode check for dirs

We could delete directories transactionally on rmdir()/unlink(), but we
don't; instead, like with regular files we wait for the VFS to call
evict().

That means that our check for directories in the deleted inodes btree is
wrong - the check should be for non-empty directories.

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