mqueuefs: Do not allow manipulation of the pseudo-dirents "." and ".."

"." and ".." names are not maintained in the mqueuefs dirent datastructure and
cannot be opened as mqueues. Creating or removing them is invalid; return
EINVAL instead of crashing.

PR: 236836
Submitted by: Torbjørn Birch Moltu <t.b.moltu AT lyse.net>
Discussed with: jilles (earlier version)