Lockless path through afs_linux_dentry_revalidate

Lockless path through afs_linux_dentry_revalidate

Permit a popular path through afs_linux_dentry_revalidate to pass without
taking a lock which it actually does not need. This affects multi-core
software-build nodes in particular, where serialization and high stat() counts
restricts useful processing to a single core.

Change-Id: I6151a1240519d9f91f6e258af71b797ce276f4e1
Reviewed-on: http://gerrit.openafs.org/3298
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>