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

This reverts commit cccc6209 which is
commit 4faeee0c upstream.

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

Bug: 161946584
Change-Id: If50dd244848a8aa70b08be347020b263c71f4a61
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>