scsi: qla2xxx: Select qpair depending on which CPU post_cmd() gets called

In current I/O path, Tx and Rx may not be processed on same CPU. This may
lead to thrashing and optimum performance may not be achieved.

Pick qpair such that Tx and Rx are processed on same CPU.

Signed-off-by: Shreyas Deodhar <sdeodhar@marvell.com>
Signed-off-by: Nilesh Javali <njavali@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>