Merge branch 'introduce-bpf_dynptr_copy-kfunc'

Mykyta Yatsenko says:

====================
introduce bpf_dynptr_copy kfunc

From: Mykyta Yatsenko <yatsenko@meta.com>

Introduce a new kfunc, bpf_dynptr_copy, which enables copying of
data from one dynptr to another. This functionality may be useful in
scenarios such as capturing XDP data to a ring buffer.
The patch set is split into 3 patches:
1. Refactor bpf_dynptr_read and bpf_dynptr_write by extracting code into
static functions, that allows calling them with no compiler warnings
2. Introduce bpf_dynptr_copy
3. Add tests for bpf_dynptr_copy

v2->v3:
  * Implemented bpf_memcmp in dynptr_success.c test, as __builtin_memcmp
  was not inlined on GCC-BPF.
====================

Link: https://patch.msgid.link/20250226183201.332713-1-mykyta.yatsenko5@gmail.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>