xfrm: rename struct xfrm_filter

iproute2 already defines a structure with that name, let's use another one to
avoid any conflict.

CC: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>