ipc: add msgget syscall wrapper

Provide ksys_msgget() wrapper to avoid in-kernel calls to this syscall.
The ksys_ prefix denotes that this function is meant as a drop-in
replacement for the syscall. In particular, it uses the same calling
convention as sys_msgget().

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.GA17492@light.dominikbrodowski.net

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

ipc/syscall.c

@@ -68,7 +68,7 @@ SYSCALL_DEFINE6(ipc, unsigned int, call, int, first, unsigned long, second,
 					   second, fifth, third);
 		}
 	case MSGGET:
-		return sys_msgget((key_t) first, second);
+		return ksys_msgget((key_t) first, second);
 	case MSGCTL:
 		return sys_msgctl(first, second, (struct msqid_ds __user *)ptr);
 
@@ -161,7 +161,7 @@ COMPAT_SYSCALL_DEFINE6(ipc, u32, call, int, first, int, second,
 		return compat_sys_msgrcv(first, ptr, second, fifth, third);
 	}
 	case MSGGET:
-		return sys_msgget(first, second);
+		return ksys_msgget(first, second);
 	case MSGCTL:
 		return compat_sys_msgctl(first, second, compat_ptr(ptr));