xfs/049: umount -d fails when kernel wins teardown race

xfs/049: umount -d fails when kernel wins teardown race

When /etc/mtab is linked to /proc/mounts and we are using mount time
created loop devices (i.e. mount -o loop), the unmount can fail
with this amazingly informative error message:

umount: /mnt/scratch/test2: filesystem was unmounted, but mount(8) failed: Invalid argument

What it actually means in this case is that the kernel tore down the
loop device when the last reference went away, and it did it so fast
that mount was not able to find it in /etc/mtab after the unmount
syscall. Hence it could not find the loop device it was supposed to
tear down and has a hissy fit.

This is simple to fix: mount does not need to tear down the loop
device as the kernel does it automatically. Remove the "-d" from
the umount command, and the test passes again.

There's quite a few other tests that also use umount -d - fix them
as well.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>