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

This reverts commit fade445f.

It breaks the Android kernel abi, but will be brought in through a
different branch to ensure it ends up in the tree properly.

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