LU-2800 autoconf: update LC_EXPORT_TRUNCATE_COMPLETE

LU-2800 autoconf: update LC_EXPORT_TRUNCATE_COMPLETE

The LC_EXPORT_TRUNCATE_COMPLETE is under the heading for RHEL4 patches
which is misleading. The test checks for:

- exported truncate_complete_page, which was never in a release
- remove_from_page_cache, exported in v2.6.35
- delete_from_page_cache, exported in v2.6.39, replaced
  removed_from_page_cache

Let's update the comments and move the test into a place in the list
where it makes sense at a glance.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: I8e53cc10d72528f9107c375cd7bb8527a27fadbd
Reviewed-on: http://review.whamcloud.com/5335
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>