When we delete a snapshot, we unlink the inode but we don't want to run
the inode_rm path - the unlink path deletes the subvolume directly,
which does everything we need. Also allowing the inode_rm path to run
was getting us "missing subvolume" errors.

There's still another bug with snapshot deletion: we need to make
snapshot deletion a multi stage process, where we unlink the root
dentry, then tear down the page cache, then delete the snapshot.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>