Revert "udp: lockless UDP_ENCAP_L2TPINUDP / UDP_GRO"

This reverts commit 8d929b6c which is
commit ac9a7f4c 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: Iabf0e545238fb27b7fd2750918ec9b079d5bba45
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>