bpf: Add typecast to bpf helpers to help BTF generation

[ Upstream commit 7c6a469e3416fa23568c2395a3faa7dd6e376dcb ]

When pahole converts dwarf to btf it emits only used types.
Wrap existing bpf helper functions into typedef and use it in
typecast to make gcc emits this type into dwarf.
Then pahole will convert it to btf.
The "btf_#name_of_helper" types will be used to figure out
types of arguments of bpf helpers.
The generated code before and after is the same.
Only dwarf and btf sections are different.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191016032505.2089704-3-ast@kernel.org


Stable-dep-of: 178c54666f9c ("bpf: Mark bpf_spin_{lock,unlock}() helpers with notrace correctly")
Signed-off-by: Sasha Levin <sashal@kernel.org>