docs/bpf: Fix sphinx warnings in BPF map docs

Fix duplicate C declaration warnings when using sphinx >= 3.1.

Reported-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/bpf/ed4dac84-1b12-5c58-e4de-93ab9ac67c09@gmail.com
Link: https://lore.kernel.org/bpf/20221122143933.91321-1-donald.hunter@gmail.com

docs/bpf: Fix sample code in MAP_TYPE_ARRAY docs

Remove mistaken & from code example in MAP_TYPE_ARRAY docs

Fixes: 1cfa97b30c5a ("bpf, docs: Document BPF_MAP_TYPE_ARRAY")
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20221115095910.86407-1-donald.hunter@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

bpf, docs: Document BPF_MAP_TYPE_ARRAY

Add documentation for the BPF_MAP_TYPE_ARRAY including kernel version
introduced, usage and examples. Also document BPF_MAP_TYPE_PERCPU_ARRAY
which is similar.

Co-developed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Maryam Tahhan <mtahhan@redhat.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/bpf/20221109174604.31673-2-donald.hunter@gmail.com