rstp: Fix memory leak reported by valgrind.

rstp: Fix memory leak reported by valgrind.

test case: 1650 RSTP Single bridge, call stacks
    hmap_insert_at (hmap.h:235)
    rstp_port_set_port_number__ (rstp.c:744)
    rstp_add_port (rstp.c:1164)
    new_bridge (test-rstp.c:123)
    test_rstp_main (test-rstp.c:514)
    ovstest_wrapper_test_rstp_main__ (test-rstp.c:714)
    ovs_cmdl_run_command (command-line.c:121)
    main (ovstest.c:132)
fix it by adding hmap_destroy() at rstp_unref()

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Co-authored-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Daniele Venturino <daniele.venturino@m3s.it>
Signed-off-by: Ben Pfaff <blp@ovn.org>