rx msgsize retry logic change

rx msgsize retry logic change

only rewrite to RX_MSGSIZE if the MTU changed. otherwise, leave well
enough alone

Change-Id: If7311c623c61064c52b42fbb131aa8ff007c3741
Reviewed-on: http://gerrit.openafs.org/2664
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit b91e7e86b429b6182d8ee23c471bef4f48af560f)
Reviewed-on: http://gerrit.openafs.org/2698
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>