Merge branch 'ss-threads' into next

Peilin Ye  says:

====================

From: Peilin Ye <peilin.ye@bytedance.com>

This patchset adds a new ss option, -T (--threads), to show thread
information.  It extends the -p (--processes) option, and should be useful
for debugging, monitoring multi-threaded applications.  Example output:

  $ ss -ltT "sport = 1234"
  State   Recv-Q  Send-Q  Local Address:Port      Peer Address:Port       Process
  LISTEN  0       100           0.0.0.0:1234           0.0.0.0:*           users:(("test",pid=2932547,tid=2932548,fd=3),("test",pid=2932547,tid=2932547,fd=3))

It implies -p i.e. it outputs all threads in the thread group, including
the thread group leader.  When -T is used, -Z and -z also show SELinux
contexts for threads.

[1-5/7] are small clean-ups for the user_ent_hash_build() function.  [6/7]
factors out logic iterating $PROC_ROOT/$PID/fd/ from user_ent_hash_build()
to make [7/7] easier.  [7/7] actually implements the feature.

====================

Signed-off-by: David Ahern <dsahern@kernel.org>