src/pmdas/linux/ipc.c: rework shmctl() argument passing
src/pmdas/linux/ipc.c: rework shmctl() argument passing

Workaround for a (bogus) buffer overrun on one platform
(vm28 x86_64 RHEL Server 6.8).

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp