LU-8116 build: Fix libmount configure message

The configure messages for libmount checking were incorrectly nested resulting
in messages like this:

checking whether build with libmount... checking libmount/libmount.h usability... no
checking libmount/libmount.h presence... no
checking for libmount/libmount.h... no
no

This patch unwinds that mistake.

Change-Id: Iec716d8152880902d3e6961cd1d3b821e37c349a
Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-on: http://review.whamcloud.com/20055
Tested-by: Jenkins
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>