x86/alternative: Add debug prints to apply_retpolines()

Make sure we can see the text changes when booting with
'debug-alternative'.

Example output:

 [ ] SMP alternatives: retpoline at: __traceiter_initcall_level+0x1f/0x30 (ffffffff8100066f) len: 5 to: __x86_indirect_thunk_rax+0x0/0x20
 [ ] SMP alternatives: ffffffff82603e58: [2:5) optimized NOPs: ff d0 0f 1f 00
 [ ] SMP alternatives: ffffffff8100066f: orig: e8 cc 30 00 01
 [ ] SMP alternatives: ffffffff8100066f: repl: ff d0 0f 1f 00

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.422273830@infradead.org

arch/x86/kernel/alternative.c

@@ -492,9 +492,15 @@ void __init_or_module noinline apply_retpolines(s32 *start, s32 *end)
 			continue;
 		}
 
+		DPRINTK("retpoline at: %pS (%px) len: %d to: %pS",
+			addr, addr, insn.length,
+			addr + insn.length + insn.immediate.value);
+
 		len = patch_retpoline(addr, &insn, bytes);
 		if (len == insn.length) {
 			optimize_nops(bytes, len);
+			DUMP_BYTES(((u8*)addr),  len, "%px: orig: ", addr);
+			DUMP_BYTES(((u8*)bytes), len, "%px: repl: ", addr);
 			text_poke_early(addr, bytes, len);
 		}
 	}