bpf: verifier: refactor kfunc specialization

bpf: verifier: refactor kfunc specialization

Move kfunc specialization (function address substitution) to later stage
of verification to support a new use case, where we need to take into
consideration whether kfunc is called in sleepable context.

Minor refactoring in add_kfunc_call(), making sure that if function
fails, kfunc desc is not added to tab->descs (previously it could be
added or not, depending on what failed).

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251026203853.135105-9-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>