compat_ioctl: add scsi_compat_ioctl

In order to move the compat handling for SCSI ioctl commands out of
fs/compat_ioctl.c into the individual drivers, we need a helper function
first to match the native ioctl handler called by sd, sr, st, etc.

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