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

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

Introducing the ksys_mknod() wrapper allows us to avoid the in-kernel
calls to sys_mknod() 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_mknod().

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

@@ -988,4 +988,13 @@ static inline long ksys_symlink(const char __user *oldname,
 	return do_symlinkat(oldname, AT_FDCWD, newname);
 }
 
+extern long do_mknodat(int dfd, const char __user *filename, umode_t mode,
+		       unsigned int dev);
+
+static inline long ksys_mknod(const char __user *filename, umode_t mode,
+			      unsigned int dev)
+{
+	return do_mknodat(AT_FDCWD, filename, mode, dev);
+}
+
 #endif