fs: add ksys_open() wrapper; remove in-kernel calls to sys_open()

Using this wrapper allows us to avoid the in-kernel calls to the
sys_open() 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_open().

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>

fs/open.c

@@ -1151,7 +1151,7 @@ COMPAT_SYSCALL_DEFINE4(openat, int, dfd, const char __user *, filename, int, fla
  */
 SYSCALL_DEFINE2(creat, const char __user *, pathname, umode_t, mode)
 {
-	return sys_open(pathname, O_CREAT | O_WRONLY | O_TRUNC, mode);
+	return ksys_open(pathname, O_CREAT | O_WRONLY | O_TRUNC, mode);
 }
 
 #endif