fs: add ksys_chdir() helper; remove in-kernel calls to sys_chdir()

Using this helper allows us to avoid the in-kernel calls to the sys_chdir()
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_chdir().

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/[e-mail]

Cc: Al Viro <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Signed-off-by: Dominik Brodowski <[e-mail]>

drivers/base/devtmpfs.c

···
 	*err = ksys_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, options);
 	if (*err)
 		goto out;
-	sys_chdir("/.."); /* will traverse into overmounted root */
+	ksys_chdir("/.."); /* will traverse into overmounted root */
 	ksys_chroot(".");
 	complete(&setup_done);
 	while (1) {

fs/open.c

···
 	return sys_faccessat(AT_FDCWD, filename, mode);
 }
 
-SYSCALL_DEFINE1(chdir, const char __user *, filename)
+int ksys_chdir(const char __user *filename)
 {
 	struct path path;
 	int error;
···
 	}
 out:
 	return error;
+}
+
+SYSCALL_DEFINE1(chdir, const char __user *, filename)
+{
+	return ksys_chdir(filename);
 }
 
 SYSCALL_DEFINE1(fchdir, unsigned int, fd)

include/linux/syscalls.h

···
 int ksys_dup(unsigned int fildes);
 int ksys_chroot(const char __user *filename);
 ssize_t ksys_write(unsigned int fd, const char __user *buf, size_t count);
+int ksys_chdir(const char __user *filename);
 
 #endif

init/do_mounts.c

···
 	if (err)
 		return err;
 
-	sys_chdir("/root");
+	ksys_chdir("/root");
 	s = current->fs->pwd.dentry->d_sb;
 	ROOT_DEV = s->s_dev;
 	printk(KERN_INFO

init/do_mounts_initrd.c

···
 	ksys_dup(0);
 	ksys_dup(0);
 	/* move initrd over / and chdir/chroot in initrd root */
-	sys_chdir("/root");
+	ksys_chdir("/root");
 	ksys_mount(".", "/", NULL, MS_MOVE, NULL);
 	ksys_chroot(".");
 	sys_setsid();
···
 	/* mount initrd on rootfs' /root */
 	mount_block_root("/dev/root.old", root_mountflags & ~MS_RDONLY);
 	sys_mkdir("/old", 0700);
-	sys_chdir("/old");
+	ksys_chdir("/old");
 
 	/* try loading default modules from initrd */
 	load_default_modules();
···
 	ksys_chroot("..");
 
 	if (new_decode_dev(real_root_dev) == Root_RAM0) {
-		sys_chdir("/old");
+		ksys_chdir("/old");
 		return;
 	}
 
-	sys_chdir("/");
+	ksys_chdir("/");
 	ROOT_DEV = new_decode_dev(real_root_dev);
 	mount_root();