compat_ioctl: bsg: add handler

bsg_ioctl() calls into scsi_cmd_ioctl() for a couple of generic commands
and relies on fs/compat_ioctl.c to handle it correctly in compat mode.

Adding a private compat_ioctl() handler avoids that round-trip and lets
us get rid of the generic emulation once this is done.

Note that bsg implements an SG_IO command that is different from the
other drivers and does not need emulation.

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