ipc/mqueue: remove unnecessary (void*) conversion

Remove unnecessary void* type casting.

Link: https://lkml.kernel.org/r/20220628021251.17197-1-yuzhe@nfschina.com
Signed-off-by: Yu Zhe <yuzhe@nfschina.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>