xdp: split code for map vs non-map redirect

The compiler does an efficient job of inlining static C functions.
Perf top clearly shows that almost everything gets inlined into the
function call xdp_do_redirect.

The function xdp_do_redirect end-up containing and interleaving the
map and non-map redirect code.  This is sub-optimal, as it would be
strange for an XDP program to use both types of redirect in the same
program. The two use-cases are separate, and interleaving the code
just cause more instruction-cache pressure.

I would like to stress (again) that the non-map variant bpf_redirect
is very slow compared to the bpf_redirect_map variant, approx half the
speed.  Measured with driver i40e the difference is:

- map     redirect: 13,250,350 pps
- non-map redirect:  7,491,425 pps

For this reason, the function name of the non-map variant of redirect
have been called xdp_do_redirect_slow.  This hopefully gives a hint
when using perf, that this is not the optimal XDP redirect operating mode.

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