ss: Factor out fd iterating logic from user_ent_hash_build()

ss: Factor out fd iterating logic from user_ent_hash_build()

We are planning to add a thread version of the -p, --process option.
Move the logic iterating $PROC_ROOT/$PID/fd/ into a new function,
user_ent_hash_build_task(), to make it easier.

Since we will use this function for both processes and threads, rename
local variables as such (e.g. from "process" to "task").

Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: David Ahern <dsahern@kernel.org>