Linux 3.19: Use mgs_iter in struct msghdr

Linux 3.19: Use mgs_iter in struct msghdr

struct msghdr gets msg_iov replaced by msg_iter.  Add a configure
test and adjust the affected code.

Reviewed-on: http://gerrit.openafs.org/11647
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit ec9a7c2db833efacfd0692c658c2d38ed9f852ba)

Change-Id: I9d873626d8997922aacf67a5a9ce7621ed904faa
Reviewed-on: http://gerrit.openafs.org/11661
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>