x86/traps: Cleanup and robustify decode_bug()

Notably, don't attempt to decode an immediate when MOD == 3.

Additionally have it return the instruction length, such that WARN
like bugs can more reliably skip to the correct instruction.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20250207122546.721120726@infradead.org