[SCSI] qla2xxx: Ensure there's enough request-queue space for passthru IOCBs.

The driver should ensure there's a sufficient number of IOCBs
to satisfy the number of scatter-gather entries specified in the
command.  Add a 'count' to the control structure, srb_ctx, to use
in qla2x00_alloc_iocbs().

Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: Chad Dupuis <chad.dupuis@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>