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

This reverts commit fade445f which is
commit 4faeee0c upstream.

It breaks the Android kernel abi, so revert it for now.  If it needs to
come back, it can do so in an abi-safe way.

Bug: 161946584
Change-Id: Id69a6eb41bd7cb59e307ad71de4331e4bee4f84a
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>