FBSD: StopListener glocking fixup

FBSD: StopListener glocking fixup

afs_osi_Sleep requires the glock (so that it can sleep on it); we
dropped the glock earlier in osi_StopListener because soshutdown and
soclose can sleep, but we must (unconditionally!) acquire it for our
loop waiting on rxk_ListenerPid.

Change-Id: Ibea3da2c5d18a2609989b547a1c587c25e3f07ff
Reviewed-on: http://gerrit.openafs.org/3549
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 28032bc1c1aaa4c25e94025a8fa9e2b4e8131ce3)
Reviewed-on: http://gerrit.openafs.org/3633