bpf: factor out fetching bpf_map from FD and adding it to used_maps list

Factor out the logic to extract bpf_map instances from FD embedded in
bpf_insns, adding it to the list of used_maps (unless it's already
there, in which case we just reuse map's index). This simplifies the
logic in resolve_pseudo_ldimm64(), especially around `struct fd`
handling, as all that is now neatly contained in the helper and doesn't
leak into a dozen error handling paths.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>