LU-7377 utils: Don't fail on plugin load for mount/tunefs

LU-7377 utils: Don't fail on plugin load for mount/tunefs

While loading mount_utils_zfs, if zfs modules aren't loaded, but
zfs plugin is present, it will return an error, this shouldn't cause
all module loading to fail.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Idad52745cdfa9d673ab9bd4afe38de4d51ae9a49
Reviewed-on: http://review.whamcloud.com/17128
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>