bpf: Resolve and cache fd_array objects at load time

The fd_array passed to BPF_PROG_LOAD carries the map and module BTF file
descriptors a program binds. The verifier reads it more than once during
a load: process_fd_array() walks it to bind the maps and BTFs, and
check_and_resolve_insns() and the kfunc BTF resolver later read it again
to resolve the program's BPF_PSEUDO_MAP_IDX* and module kfunc refs.

For signed BPF, we need these upfront in memory, thus resolve each fd to
its object once and cache it by fd_array index, then bind that cached
object for the rest of the load. env->fd_array becomes a small per-slot
{map, btf} cache rather than a bpfptr_t; every later reference is then
an in-bounds lookup of an already-resolved object, and an index outside
the cache is rejected instead of read from user memory:

  - continuous (fd_array_cnt given): the caller declares the length and
    every entry is resolved and bound up front (used also by the BPF
    signed loader)

  - sparse (no fd_array_cnt): left as the legacy path with no fd_array
    cache; each reference reads its fd from the caller's fd_array and
    resolves it on the spot. Deduplication in used_maps and the kfunc BTF
    table keeps this correct, and only unsigned programs use this shape.

Split these into separate helpers to make it easier to follow.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Anton Protopopov <a.s.protopopov@gmail.com>
Link: https://lore.kernel.org/bpf/20260708075343.358712-2-daniel@iogearbox.net
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>