xfstests: loop devices vs umount stupidity

xfstests: loop devices vs umount stupidity

Unmounting a fileystem mounted on a loop device doesn't always tear
down the loop device. Its racy, and it causes tests to randomly
fail.

To avoid that, we have to use umount -d to ensure that we destroy
loop devices under filesystems in case the kernel doesn't tear it
down automatically to prevent the test from failing.  However, if
the kernel does tear it down automatically, umount now issues a
warning that it couldn't tear down the loop device because it
couldn't find it, and that causes the test to fail. *facepalm*

So, convert all the loop device unmounts to use -d, and direct the
output of all of them to /dev/null.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>