net: Decrease lock contention within the TCP accept case

This is a backport of from freebsd:

   commit 5c066f64123bc86c10537ae180565c8766e3cc2c
   Author: gnn <gnn@FreeBSD.org>
   Date:   Tue Jan 28 20:28:32 2014 +0000

       Decrease lock contention within the TCP accept case by removing
       the INP_INFO lock from tcp_usr_accept.  As the PR/patch states
       this was following the advice already in the code.
       See the PR below for a full disucssion of this change and its
       measured effects.

       PR:         183659
       Submitted by:       Julian Charbo

A modification is made compared to original patch. In OSv, we call
soisconnected() before INP_UNLOCK. This is becasue in 79005e32 (net:
remove socket lock, use protocol layer connection lock instead), an
extra SOCK_UNLOCK(so) was added to soisconnected(), ...

Signed-off-by: Asias He <asias@cloudius-systems.com>