LU-4769 tests: conf-sanity/38 fix regression

stop_mds on zfs exports the pool.  Use mount_fstype that accounts for
importing pool if needed.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Id3fbfc61af032aaaca1030a5927bd509725a0f21
Reviewed-on: http://review.whamcloud.com/9669
Tested-by: Jenkins
Reviewed-by: Isaac Huang <he.huang@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>