bpf: save subprogram name in bpf_subprog_info

bpf: save subprogram name in bpf_subprog_info

Subprogram name can be computed from function info and BTF, but it is
convenient to have the name readily available for logging purposes.
Update comment saying that bpf_subprog_info->start has to be the first
field, this is no longer true, relevant sites access .start field
by it's name.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260410-patch-set-v4-2-5d4eecb343db@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>