fs: add do_faccessat() helper and ksys_access() wrapper; remove in-kernel calls to syscall

Using the fs-internal do_faccessat() helper allows us to get rid of
fs-internal calls to the sys_faccessat() syscall.

Introducing the ksys_access() wrapper allows us to avoid the in-kernel
calls to the sys_access() 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_access().

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>

include/linux/syscalls.h

@@ -1014,4 +1014,11 @@ static inline int ksys_chmod(const char __user *filename, umode_t mode)
 	return do_fchmodat(AT_FDCWD, filename, mode);
 }
 
+extern long do_faccessat(int dfd, const char __user *filename, int mode);
+
+static inline long ksys_access(const char __user *filename, int mode)
+{
+	return do_faccessat(AT_FDCWD, filename, mode);
+}
+
 #endif