bpf: declare a few utility functions as internal api

bpf: declare a few utility functions as internal api

Namely, rename the following functions and add prototypes to
bpf_verifier.h:
- find_containing_subprog -> bpf_find_containing_subprog
- insn_successors         -> bpf_insn_successors
- calls_callback          -> bpf_calls_callback
- fmt_stack_mask          -> bpf_fmt_stack_mask

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250918-callchain-sensitive-liveness-v3-4-c3cd27bacc60@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>