sock.h is pretty heavily used (5k objects rebuilt on x86 after
it's touched). We can drop the include of filter.h from it and
add a forward declaration of struct sk_filter instead.
This decreases the number of rebuilt objects when bpf.h
is touched from ~5k to ~1k.

There's a lot of missing includes this was masking. Primarily
in networking tho, this time.
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Acked-by: NMarc Kleine-Budde <mkl@pengutronix.de>
Acked-by: NFlorian Fainelli <f.fainelli@gmail.com>
Acked-by: NNikolay Aleksandrov <nikolay@nvidia.com>
Acked-by: NStefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/bpf/20211229004913.513372-1-kuba@kernel.org