bpf: Allow helpers to accept pointers with a fixed size

Before this commit, the BPF verifier required ARG_PTR_TO_MEM arguments
to be followed by ARG_CONST_SIZE holding the size of the memory region.
The helpers had to check that size in runtime.

There are cases where the size expected by a helper is a compile-time
constant. Checking it in runtime is an unnecessary overhead and waste of
BPF registers.

This commit allows helpers to accept pointers to memory without the
corresponding ARG_CONST_SIZE, given that they define the memory region
size in struct bpf_func_proto and use ARG_PTR_TO_FIXED_SIZE_MEM type.

arg_size is unionized with arg_btf_id to reduce the kernel image size,
and it's valid because they are used by different argument types.

Signed-off-by: Maxim Mikityanskiy <[email protected]>
Reviewed-by: Tariq Toukan <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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