Revert "tcp: deny tcp_disconnect() when threads are waiting"

This reverts commit 32e9a9ee which is
commit 4faeee0c upstream.

It breaks the Android kernel abi, and if it is needed can be brought
back in an ABI-safe way.

Bug: 161946584
Change-Id: I228195563c82462203d9aa93d7287d72481204c6
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>