overlay/036: filter busy mount message

overlay/036: filter busy mount message

util-linux v2.30 changed error message of a busy mount and caused
overlay/036 to fail. e.g.

 - mount: <device> is already mounted or <mountpoint> busy
 + mount: <mountpoint>: <device> already mounted or mount point busy.

Filter the mount output by a newly introduced _filter_busy_mount
into a unified format.

   mount: device already mounted or mount point busy

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>