LU-718 mds: fix mds-max-threads configure parameter

LU-718 mds: fix mds-max-threads configure parameter

Commit 648b69c2c9 renamed MDT_MAX_THREADS to MDS_MAX_THREADS, but
didn't update the configuration parameter for setting this value
at build time.  Fortunately, the configure parameter itself has
the correct name to begin with, so no build-visible changes needed.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I21dc22e470e7e3788ebef8009643b5c7a98bc220
Reviewed-on: http://review.whamcloud.com/5723
Tested-by: Hudson
Reviewed-by: wangdi <di.wang@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>