bpf/helpers: Refactor bpf_dynptr_read and bpf_dynptr_write

Refactor bpf_dynptr_read and bpf_dynptr_write helpers: extract code
into the static functions namely __bpf_dynptr_read and
__bpf_dynptr_write, this allows calling these without compiler warnings.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250226183201.332713-2-mykyta.yatsenko5@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>