compat_ioctl: move sys_compat_ioctl() to ioctl.c

The rest of the fs/compat_ioctl.c file is no longer useful now,
so move the actual syscall as planned.

Reviewed-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>