macos: fix race in afs_root

macos: fix race in afs_root

same race on PutVCache in afs_root as we had on other platforms,
for instance FreeBSD. use a local variable instead to avoid the race.
additionally, make sure we end up with the root flagged VROOT.

Reviewed-on: http://gerrit.openafs.org/5278
Tested-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit ad6e19331e4f3ec6566dc224f1e1c943a69c62bb)

Change-Id: I74d957ac47e10e7a3ddbcd0cadcd6a69deb7df99
Reviewed-on: http://gerrit.openafs.org/5286
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>