The memset for zeroing work completions had been unconditional.

This patch removes the memset and moves the zeroing into the work
completion with a more explicit field by field set.  With this patch,
non-ONLY/non-LAST packets will avoid the overhead since they will not
generate a completion.
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>