fs: add ksys_fchmod() and do_fchmodat() helpers and ksys_chmod() wrapper; remove in-kernel calls to syscall

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

Introducing the ksys_fchmod() helper and the ksys_chmod() wrapper allows
us to avoid the in-kernel calls to the sys_fchmod() and sys_chmod()
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_fchmod() and sys_chmod().

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>

init/initramfs.c

@@ -344,7 +344,7 @@ static int __init do_name(void)
 
 			if (wfd >= 0) {
 				sys_fchown(wfd, uid, gid);
-				sys_fchmod(wfd, mode);
+				ksys_fchmod(wfd, mode);
 				if (body_len)
 					sys_ftruncate(wfd, body_len);
 				vcollected = kstrdup(collected, GFP_KERNEL);
@@ -354,14 +354,14 @@ static int __init do_name(void)
 	} else if (S_ISDIR(mode)) {
 		ksys_mkdir(collected, mode);
 		sys_chown(collected, uid, gid);
-		sys_chmod(collected, mode);
+		ksys_chmod(collected, mode);
 		dir_add(collected, mtime);
 	} else if (S_ISBLK(mode) || S_ISCHR(mode) ||
 		   S_ISFIFO(mode) || S_ISSOCK(mode)) {
 		if (maybe_link() == 0) {
 			ksys_mknod(collected, mode, rdev);
 			sys_chown(collected, uid, gid);
-			sys_chmod(collected, mode);
+			ksys_chmod(collected, mode);
 			do_utime(collected, mtime);
 		}
 	}