Linux: 2.6.38: Adjust for permission inode operation changes

Linux: 2.6.38: Adjust for permission inode operation changes

The permission i_op has a new signature with a flags argument, and
must now deal with RCU path walking.
- Fix existing configure test for this i_op, it succeeds when it
shouldn't
- Add a new configure test for the new signature
- Make our permission i_op "RCU-walk aware" - return ECHILD if
called in that mode

Reviewed-on: http://gerrit.openafs.org/3770
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 5bcc0ea735ea519298c98b46c66bf1326cdee5e4)

Change-Id: Iebfab65b3442ea286873e6af96f1c05c98e5d126
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/3997
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Russ Allbery <rra@stanford.edu>
(cherry picked from commit 29855d9dc5d117ba106bdb0dd9a9bc6e0592c19c)