ipc: add semtimedop syscall/compat_syscall wrappers

Provide ksys_semtimedop() and compat_ksys_semtimedop() 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_semtimedop() and
compat_sys_semtimedop().

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/util.h

@@ -235,4 +235,17 @@ static inline int compat_ipc_parse_version(int *cmd)
 #endif
 }
 #endif
+
+/* for __ARCH_WANT_SYS_IPC */
+long ksys_semtimedop(int semid, struct sembuf __user *tsops,
+		     unsigned int nsops,
+		     const struct timespec __user *timeout);
+
+/* for CONFIG_ARCH_WANT_OLD_COMPAT_IPC */
+#ifdef CONFIG_COMPAT
+long compat_ksys_semtimedop(int semid, struct sembuf __user *tsems,
+			    unsigned int nsops,
+			    const struct compat_timespec __user *timeout);
+#endif /* CONFIG_COMPAT */
+
 #endif