Revert "ipv4: ip_tunnel: Unmask upper DSCP bits in ip_md_tunnel_xmit()"

This reverts commit d6ff1c86 which is
commit c34cfe72 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: Id1deb68834b6b9756b96fd67ce3ec20a3c9f3321
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>