ipc: add msgsnd syscall/compat_syscall wrappers

Provide ksys_msgsnd() and compat_ksys_msgsnd() 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_msgsnd() and compat_sys_msgsnd().

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>
diff --git a/ipc/syscall.c b/ipc/syscall.c
index 0228c7a..77a883e 100644
--- a/ipc/syscall.c
+++ b/ipc/syscall.c
@@ -46,7 +46,7 @@
 	}
 
 	case MSGSND:
-		return sys_msgsnd(first, (struct msgbuf __user *) ptr,
+		return ksys_msgsnd(first, (struct msgbuf __user *) ptr,
 				  second, third);
 	case MSGRCV:
 		switch (version) {
@@ -142,7 +142,7 @@
 		return compat_ksys_semctl(first, second, third, pad);
 
 	case MSGSND:
-		return compat_sys_msgsnd(first, ptr, second, third);
+		return compat_ksys_msgsnd(first, ptr, second, third);
 
 	case MSGRCV: {
 		void __user *uptr = compat_ptr(ptr);