[SCSI] mpt2sas: Fix for kernel panic when driver loads with HBA connected to non LUN 0 configured expander

With some enclosures when LUN 0 is not created but LUN 1 or LUN X is created
then SCSI scan procedure calls target_alloc, slave_alloc call back functions
for LUN 0 and slave_destory() for same LUN 0.

In these kind of cases within slave_destroy, pointer to scsi_target in
_sas_device structure is set to NULL, following which when slave_alloc for
LUN 1 is called then starget would not be set properly for this LUN.
So, scsi_target pointer pointing to NULL value would lead to a crash later
in the discovery procedure.

To solve this issue set the sas_device's scsi_target pointer to scsi_device's
scsi_target if it is NULL earlier in slave_alloc callback function.
Signed-off-by: Sreekanth Reddy <Sreekanth.Reddy@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>