Linux: 2.6.38: deal with dcache_lock removal

Linux: 2.6.38: deal with dcache_lock removal

dcache_lock is gone in 2.6.38, and some of the vfs locking rules
have changed.

Of interest for openafs:
- inode->i_lock protects the d_alias list
- dentry->d_lock protects d_unhashed()

Add a new configure test for dcache_lock, and replace its use by
the appropriate new lock(s).

Reviewed-on: http://gerrit.openafs.org/3771
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 2eca7aef7b2940e4ef5f9901ce28481af6edb6dd)

Change-Id: Ic544db9bcf820f01f23227b71684847d742fa41f
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/3998
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 9e614fafad635704acd7958283903ee2f88dc1c6)