ipc: add shmctl syscall/compat_syscall wrappers

ipc: add shmctl syscall/compat_syscall wrappers

Provide ksys_shmctl() and compat_ksys_shmctl() wrappers to avoid in-kernel
calls to these syscalls. The ksys_ prefix denotes that these functions are
meant as a drop-in replacement for the syscalls. In particular, they use
the same calling convention as sys_shmctl() and compat_sys_shmctl().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527[email protected]

Cc: Al Viro <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: Dominik Brodowski <[email protected]>