cache bypass: don't define iovecp for UKERNEL

cache bypass: don't define iovecp for UKERNEL

iovecp is defined but not used for UKERNEL.  Define it conditionally
to avoid gcc warnings and --enable-checking failure.

Reviewed-on: http://gerrit.openafs.org/5650
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 547d94edd3affb5f377cd1b3db39c46ca0cf5aec)

Change-Id: I700b82173b5c2435a716aaf10541e1583f2655f5
Reviewed-on: http://gerrit.openafs.org/6316
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>