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

This reverts commit 7d6e8d7e 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: I8d0ac023a1d1b06f07cdacdb5f60fb52e7ad782e
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>