bpf: sync bpf.h to tools/

Sync updated documentation for bpf_redirect_map.

Sync the bpf_tcp_gen_syncookie helper function definition with the one
in tools/uapi.

Signed-off-by: Petar Penkov <ppenkov@google.com>
Reviewed-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>