macos: reset next vcache pointer after reacquiring xvcache

macos: reset next vcache pointer after reacquiring xvcache

dropping the xvcache lock means that things can change out from under
us. in case they do, reset the next vcache pointer before looping

Reviewed-on: http://gerrit.openafs.org/5184
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit e8607d8d250fb41fcf4e60a09917f971c4d6104a)

Change-Id: I2ad487e348f84cd7ced884c920230e3371fce902
Reviewed-on: http://gerrit.openafs.org/5186
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>