bash completion for storage option `dm.min_free_space`

Signed-off-by: Harald Albers <github@albersweb.de>
(cherry picked from commit eae4e84c6618e2633347477d788eb351529b0361)

contrib/completion/bash/docker

@@ -887,6 +887,7 @@ _docker_daemon() {
 				dm.fs
 				dm.loopdatasize
 				dm.loopmetadatasize
+				dm.min_free_space
 				dm.mkfsarg
 				dm.mountopt
 				dm.override_udev_sync_check