Fix datasets held open after tests

Fix for post-testing error:

`cannot destroy 'tmp.uwIwWzoNp4/tmp': dataset is busy`

Thanks to @nalind for the fix.

Signed-off-by: Chris Evich <cevich@redhat.com>

contrib/cirrus/build_and_test.sh

@@ -70,6 +70,8 @@ case $TEST_DRIVER in
         trap "zfs destroy -Rf $zpool/tmp; zpool destroy -f $zpool; rm -f $tmpfile" EXIT
         zfs create $zpool/tmp
         TMPDIR="/$zpool/tmp" showrun make STORAGE_DRIVER=$TEST_DRIVER local-test-integration local-test-unit
+        # Ensure no datasets are held open prior to `zfs destroy` trap.
+        kill $(lsns -J -t mnt --output-all | jq '.namespaces[]|select(.command=="sleep 1000s").pid')
         ;;
     *)
         die "Unknown/Unsupported \$TEST_DRIVER=$TEST_DRIVER (see .cirrus.yml and $(basename $0))"