btrfs: fix test failures after commit 27d077ec0bda

btrfs: fix test failures after commit 27d077ec0bda

Commit 27d077ec0bda (common: use mount/umount helpers everywhere) made
a few btrfs tests fail (btrfs/003 and btrfs/011). These tests create
filesystems with multiple devices and test the device replace feature,
which need to unmount using the mount path ($SCRATCH_MNT) because
unmounting using one of the devices as an argument ($SCRATCH_DEV) does
not always work - after replace operations we get in /proc/mounts a
device other than $SCRATCH_DEV associated with the mount point
$SCRATCH_MNT (this is mentioned in a comment at btrfs/011 for example),
so we need to pass that other device to the umount program or pass it the
mount point.
Fix this by making _sctatch_unmount() pass $SCRATCH_MNT to umount instead
of $SCRATCH_DEV (when the filesystem being tested is btrfs).

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>