selftest: add a reuseaddr test

This is to test for a regression introduced by

b9470c27

 ("inet: kill smallest_size and smallest_port")

which introduced a problem with reuseaddr and bind conflicts.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>