bpf: Apply CO-RE relocations before subprogram validation

check_subprogs() verifies that each subprogram ends in an exit or an
unconditional jump before in-kernel CO-RE relocations are applied. An
unresolved relocation can then replace that terminal instruction with an
invalid helper call. The resulting fall-through into another subprogram
breaks the CFG invariant used by postorder and stack liveness analysis,
which can write past their per-subprogram arrays.

Apply CO-RE relocations immediately after preparing the program BTF, before
subprogram discovery and validation. Keep func_info and line_info validation
after subprogram discovery because those records depend on the complete
subprogram layout.

Reject an ldimm64 first slot at the end of the instruction stream before
CO-RE can inspect its missing second slot. check_subprogs() previously
rejected this form before relocation processing because it is not a valid
subprogram terminator. Moving CO-RE ahead of check_subprogs() removes that
implicit protection, so perform an explicit check before applying
relocations.

Include core_relo_cnt when deciding whether to prepare program BTF. A load
that supplied only CO-RE relocation metadata previously skipped both BTF
setup and relocation processing.

Fixes: fbd94c7afcf9 ("bpf: Pass a set of bpf_core_relo-s to prog_load command.")
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20260917233222.2542500-5-memxor@gmail.com
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

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