fs: add ksys_chroot() helper; remove-in kernel calls to sys_chroot()

Using this helper allows us to avoid the in-kernel calls to the
sys_chroot() 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_chroot().

In the near future, the fs-external callers of ksys_chroot() should be
converted to use kern_path()/set_fs_root() directly. Then ksys_chroot()
can be moved within sys_chroot() again.

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: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

drivers/base/devtmpfs.c

@@ -387,7 +387,7 @@ static int devtmpfsd(void *p)
 	if (*err)
 		goto out;
 	sys_chdir("/.."); /* will traverse into overmounted root */
-	sys_chroot(".");
+	ksys_chroot(".");
 	complete(&setup_done);
 	while (1) {
 		spin_lock(&req_lock);