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

This reverts commit c2251ce0.

It breaks the Android KABI and will be brought back at a later time when
it is safe to do so.

Bug: 161946584
Change-Id: Ie84b5bfe5abc526a6117e5ee320a19ecf04b89bb
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>