ss: fix directory leak when -T option is used

ss: fix directory leak when -T option is used

To get information about threads used in a process, the /proc/$PID/task
directory content is analyzed by ss code. However, the opened 'dirent'
object is not closed after use, leading to memory leaks. Add missing
closedir call in 'user_ent_hash_build' to avoid it.

Detected by valgrind: "valgrind ./misc/ss -T"

Fixes: e2267e68b9b5 ("ss: Introduce -T, --threads option")
Signed-off-by: Maxim Petrov <mmrmaximuzz@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>