ipc/mqueue: add missing sparse annotation

Link: http://lkml.kernel.org/r/20170128235704.45302-1-luc.vanoostenryck@gmail.com


Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>