[SCSI] qla2xxx: Consolidation of SRB processing.

Rework the structures related to SRB processing to minimize the memory
allocations per I/O and manage resources associated with and completions
from common routines.

Signed-off-by: Giridhar Malavali <giridhar.malavali@qlogic.com>
Signed-off-by: Chad Dupuis <chad.dupuis@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>