LU-9137 utils: replace readdir_r with readdir

LU-9137 utils: replace readdir_r with readdir

Originally readdir_r was created to handle the case of readdir
not being reentrant. The readdir_r implementation has several
flaws and modern platforms have a glibc version were readdir
is reentrant. Because of this platforms are now marking
readdir_r as obsolete which breaks our lustre build. This
patch migrates us to readdir instead.

Change-Id: I939d5a402e6c73c56615972b2b3c6f0a21b59b55
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/27276
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Jenkins
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>