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

This reverts commit 71787c66 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: I18ef204d3c165adca98146fa07d2fd25671d7b42
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>