mdconfig: fix cleanup in the attach_size_rounddown test

Sponsored by:	Axcient

(cherry picked from commit d1bd097d52)

sbin/mdconfig/tests/mdconfig_test.sh

@@ -291,7 +291,7 @@ attach_size_rounddown_body()
 	    -x "mdconfig -r -u ${md#md} -s ${ms2}b"
 	check_diskinfo "$md" 16384 2 $ss
 }
-attach_size_rounddown()
+attach_size_rounddown_cleanup()
 {
 	cleanup_common
 }