LINUX: Use struct vfs_path on RHEL5

LINUX: Use struct vfs_path on RHEL5

Some revisions of the kernel from RHEL5 (2.6.18-308.* and possibly
others) renamed 'struct path' to 'struct vfs_path'. So, use
'struct vfs_path' when it exists.

This introduces the afs_linux_path_t typedef, which is defined as
either a struct path, or struct vfs_path.

Reviewed-on: http://gerrit.openafs.org/8019
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 74c1881dff1593f4a8920ba8d8a2400760899fa5)

Change-Id: I6eaf17ebe9a35b345c1fd5cebd490f8095a1b22b
Reviewed-on: http://gerrit.openafs.org/8789
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>