linux: 2.6.38: Make d_revalidate RCU-walk aware

linux: 2.6.38: Make d_revalidate RCU-walk aware

Signal to the vfs that we don't support path RCU walk in
d_revalidate.  It will call back in non-RCU mode.

Reviewed-on: http://gerrit.openafs.org/3759
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 52556d50298e4a636f07e9a3cf0b6557c882c82d)

Change-Id: I6662782068e317a36ca70caa0d09665ff2183c2d
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/3995
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Russ Allbery <rra@stanford.edu>
(cherry picked from commit 9a91534004f853de427a661347fe029218ea9ac2)