FBSD: close race in afs_root

FBSD: close race in afs_root

Previously, we called afs_PutVCache(afs_globalVp) directly.  This
is unsafe because PutVCache acquires locks which can sleep, losing
the serialization of the GLOCK.  In rare circumstances, this can
result in two threads simultaneously making that call, and the
second one would panic in vputx() with a negative refcount.
Close the race by using a local variable for the afs_PutVCache()
calls, applying the change to afs_globalVp before dropping the GLOCK.
While here, fix up other race conditions.

Change-Id: Id62f27d4469a96c9becd9b031125bd6c910c3e9b
Reviewed-on: http://gerrit.openafs.org/3275
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>