resolve_btfids: Factor out load_btf()

resolve_btfids: Factor out load_btf()

Increase the lifetime of parsed BTF in resolve_btfids by factoring
load_btf() routine out of symbols_resolve() and storing the base_btf
and btf pointers in the struct object.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20251219181321.1283664-3-ihor.solodrai@linux.dev