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.

Change-Id: I88c31125dbc1ab2998a521e3c57ed532bf2d5bb7
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>