xdp: explicit inline __xdp_map_lookup_elem

The compiler chooses to not-inline the function __xdp_map_lookup_elem,
because it can see that it is used by both Generic-XDP and native-XDP
do redirect calls (xdp_do_generic_redirect_map and xdp_do_redirect_map).

The compiler cannot know that this is a bad choice, as it cannot know
that a net device cannot run both XDP modes (Generic or Native) at the
same time.  Thus, mark this function inline, even-though we normally
leave this up-to the compiler.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>