LU-6394 all: fix compilation errors with FORTIFY_SOURCE

LU-6394 all: fix compilation errors with FORTIFY_SOURCE

When Lustre is configured with CFLAGS="-D_FORTIFY_SOURCE=2 -O2" on
Centos 6, the compilation will fails with errors such as this one:

 cacheio.c: In function ‘qword_printhex’:
 cacheio.c:174: error: ignoring return value of ‘fwrite’, declared
   with attribute warn_unused_result

Signed-off-by: frank zago <fzago@cray.com>
Change-Id: Ie06fd5a26b62daf62bfd0133a2d7ebc66ece5be6
Reviewed-on: http://review.whamcloud.com/14126
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>