ipc: add semget syscall wrapper

Provide ksys_semget() 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_semget().

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 84d6a76..21fcdf0 100644
--- a/ipc/syscall.c
+++ b/ipc/syscall.c
@@ -35,7 +35,7 @@ SYSCALL_DEFINE6(ipc, unsigned int, call, int, first, unsigned long, second,
 				       (const struct timespec __user *)fifth);
 
 	case SEMGET:
-		return sys_semget(first, second, third);
+		return ksys_semget(first, second, third);
 	case SEMCTL: {
 		unsigned long arg;
 		if (!ptr)
@@ -132,7 +132,7 @@ COMPAT_SYSCALL_DEFINE6(ipc, u32, call, int, first, int, second,
 		return compat_ksys_semtimedop(first, compat_ptr(ptr), second,
 						compat_ptr(fifth));
 	case SEMGET:
-		return sys_semget(first, second, third);
+		return ksys_semget(first, second, third);
 	case SEMCTL:
 		if (!ptr)
 			return -EINVAL;