bpf: Export bpf_dynptr_get_size()

Export bpf_dynptr_get_size(), so that kernel code dealing with eBPF dynamic
pointers can obtain the real size of data carried by this data structure.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Acked-by: KP Singh <kpsingh@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220920075951.929132-6-roberto.sassu@huaweicloud.com


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