Revert "mptcp: fix lockless access in subflow ULP diag"

This reverts commit 8affdbb3 which is
commit b8adb69a upstream.

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

Bug: 161946584
Change-Id: Ib22d4d7d600994c79e87c929870ba31cc5a4aad3
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>