LINUX: Fix oops during negative dentry caching

LINUX: Fix oops during negative dentry caching

Commit 2e9dcc069904aaa434787eec53c6f9821911cbab reinstated negative
dentry caching, but introduced an oops when fakestat is in use.  Be sure
the GLOCK is held when looking up the parent vcache dv when the parent
is a mount point and fakestat is in use, since the calls to do the
lookup require the GLOCK to be held.

Change-Id: I6c47fbf53280400bf40271b1ff2837bd7c6dc69e
Reviewed-on: http://gerrit.openafs.org/12019
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>