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

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

Introducing the ksys_mkdir() wrapper allows us to avoid the in-kernel calls
to the sys_mkdir() 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_mkdir().

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

@@ -972,4 +972,11 @@ static inline long ksys_rmdir(const char __user *pathname)
 	return do_rmdir(AT_FDCWD, pathname);
 }
 
+extern long do_mkdirat(int dfd, const char __user *pathname, umode_t mode);
+
+static inline long ksys_mkdir(const char __user *pathname, umode_t mode)
+{
+	return do_mkdirat(AT_FDCWD, pathname, mode);
+}
+
 #endif