Revert "sock_diag: allow concurrent operation in sock_diag_rcv_msg()"

This reverts commit bba7266d which is
commit 86e8921d 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: Id18398903442336a49229e944ee452757c111aa9
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>