bpf: make function xdp_do_generic_redirect_map() static

The function xdp_do_generic_redirect_map() is only used in this file, so
make it static.

Clean up sparse warning:
net/core/filter.c:2687:5: warning: no previous prototype
for 'xdp_do_generic_redirect_map' [-Wmissing-prototypes]

Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>