vfs: dentry_unhash immediately prior to rmdir

This presumes that there is no reason to unhash a dentry if we fail because
it is a mountpoint or the LSM check fails, and that the LSM checks do not
depend on the dentry being unhashed.

Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>