fs: add ksys_ioctl() helper; remove in-kernel calls to sys_ioctl()

Using this helper allows us to avoid the in-kernel calls to the
sys_ioctl() 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_ioctl().

After careful review, at least some of these calls could be converted
to do_vfs_ioctl() in future.

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>

init/do_mounts_initrd.c

@@ -110,7 +110,7 @@ static void __init handle_initrd(void)
 		if (fd < 0) {
 			error = fd;
 		} else {
-			error = sys_ioctl(fd, BLKFLSBUF, 0);
+			error = ksys_ioctl(fd, BLKFLSBUF, 0);
 			ksys_close(fd);
 		}
 		printk(!error ? "okay\n" : "failed\n");