rx: Use a structure for the xmit list

rx: Use a structure for the xmit list

Switch to using a structure to hold the xmit list so that it's a
little bit clearer what the rxi_SendXmitList function is actually doing

(cherry picked from commit ad3b5997c6da17927312f4618d8bb4f7fc542a64)
Reviewed-on: http://gerrit.openafs.org/3134
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Change-Id: I6534f5c40e890af916752bf2647a14f9c386e004
Reviewed-on: http://gerrit.openafs.org/4917
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>