bpf, doc: Fix patchwork URL to point to kernel.org instance

Follow-up on ebb034b1 ("bpf: Migrate from patchwork.ozlabs.org
to patchwork.kernel.org.") in order to fix up the patchwork URL (Q)
in the MAINTAINERS file for BPF subsystem.

While at it, also add the official website (W) entry.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/f73ae01c7e6f9cf0a3890f2ca988a8e69190c50b.1603223852.git.daniel@iogearbox.net