[SCSI] gdth: Allocate sense_buffer to prevent NULL pointer dereference

Fix NULL pointer dereference during execution of Internal commands,
where gdth only allocates scp, but not scp->sense_buffer. The rest of
the code assumes that sense_buffer is allocated, which leads to a kernel
oops e.g. on reboot (during cache flush).

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>