[SCSI] libiscsi: fix senselen calculation

Yanling Qi, noted that when the sense data length of
a check-condition is greater than 0x7f (127), senselen = (data[0] << 8)
| data[1] will become negative. It causes different kinds of panics from
GPF, spin_lock deadlock to spin_lock recursion.

We were also swapping this value on big endien machines.

This patch fixes both issues by using be16_to_cpu().

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>