bpf: Report Program Structure CFG errors

Augment selected whole-program and subprogram CFG validation failures with
Program Structure reports. These errors are structural rather than
path-dependent, so the reports focus on source and instruction context
instead of causal history.

Cover direct and indirect jumps outside the program or current subprogram,
unprivileged backedges, missing and out-of-range jump tables, targets in the
second half of an ldimm64, unreachable instructions, subprogram fallthrough,
and recursive bpf2bpf call graph edges.

Format long jump-range reasons directly in diagnostics.c, and keep the
fallthrough suggestion aligned with the verifier check by suggesting exit or
explicit jumps.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20260815064612.378577-14-memxor@gmail.com
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

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