LU-6215 libcfs: shrinker code needs atomic.h

LU-6215 libcfs: shrinker code needs atomic.h

In the latest linux kernel code struct shrinker now
includes a new atomic_long_t so atomic.h is required
for the libcfs autoconf test will pass.

Change-Id: I7b5a5cc030bc8e8fc4cbd0c47bc75f73b07742f7
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: http://review.whamcloud.com/14662
Tested-by: Jenkins
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Thomas Stibor <t.stibor@gsi.de>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>