fs: compat: Remove warning from COMPATIBLE_IOCTL

cmd in COMPATIBLE_IOCTL is always a u32, so cast it so there isn't a
warning about an overflow in XFORM.

From: Mark Charlebois <charlebm@gmail.com>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
Signed-off-by: Behan Webster <behanw@converseincode.com>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>