LINUX vcache lock ordering in afs_linux_readdir

LINUX  vcache lock ordering in afs_linux_readdir

Normalize shared and exclusive lock operations.  Take the lock
exclusive immediately, since the code assumes a write lock if
the vcache state is in flux or the entry is being fetched, releasing
-write- rather than shared, since we do not hold a shared lock.

Change-Id: Icbffdf21c6fc7929483589e87ffe9131834c79b4
Reviewed-on: http://gerrit.openafs.org/5309
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementix.org>