bpf: add kfuncs and helpers support for file dynptrs

bpf: add kfuncs and helpers support for file dynptrs

Add support for file dynptr.

Introduce struct bpf_dynptr_file_impl to hold internal state for file
dynptrs, with 64-bit size and offset support.

Introduce lifecycle management kfuncs:
  - bpf_dynptr_from_file() for initialization
  - bpf_dynptr_file_discard() for destruction

Extend existing helpers to support file dynptrs in:
  - bpf_dynptr_read()
  - bpf_dynptr_slice()

Write helpers (bpf_dynptr_write() and bpf_dynptr_data()) are not
modified, as file dynptr is read-only.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251026203853.135105-8-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>