Linux: Fix fallout from path_lookup commit

Linux: Fix fallout from path_lookup commit

Fix a few issues with the recent commit to deal withg the removal
of path_lookup, spotted on RHEL 5:
- the configure tests needs fs.h to be included before namei.h, to
get the definition of struct inode
- we need to avoid the use of struct path unless its needed; on
older kernels the structure doesn't exist

Change-Id: I6251a96a371a50548dcafc70d94e91b52fc2922a
Reviewed-on: http://gerrit.openafs.org/4382
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>