f 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>
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