bpf: add find_containing_subprog() utility function

Add a utility function, looking for a subprogram containing a given
instruction index, rewrite find_subprog() to use this function.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20241210041100.1898468-2-eddyz87@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>