bpf: flowlabel in bpf_fib_lookup should be flowinfo

As Michal noted the flow struct takes both the flow label and priority.
Update the bpf_fib_lookup API to note that it is flowinfo and not just
the flow label.

Cc: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>