bpf/helpers: Introduce bpf_dynptr_copy kfunc

bpf/helpers: Introduce bpf_dynptr_copy kfunc

Introducing bpf_dynptr_copy kfunc allowing copying data from one dynptr to
another. This functionality is useful in scenarios such as capturing XDP
data to a ring buffer.
The implementation consists of 4 branches:
  * A fast branch for contiguous buffer capacity in both source and
destination dynptrs
  * 3 branches utilizing __bpf_dynptr_read and __bpf_dynptr_write to copy
data to/from non-contiguous buffer

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250226183201.332713-3-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>