x86/boot: Add a message to early NMI handler

Add a message to help diagnosis for the case if someone wants to know
what happened to NMIs during very early boot.

Signed-off-by: Jun'ichi Nomura <junichi.nomura@nec.com>
Suggested-by: Borislav Petkov <bp@alien8.de>
Link: https://lore.kernel.org/lkml/20231130103339.GCZWhlA196uRklTMNF@fat_crate.local/

diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
index d040080d7edb..f2858a373099 100644
--- a/arch/x86/boot/compressed/ident_map_64.c
+++ b/arch/x86/boot/compressed/ident_map_64.c
@@ -390,4 +390,5 @@ void do_boot_page_fault(struct pt_regs *regs, unsigned long error_code)
 void do_boot_nmi_trap(struct pt_regs *regs, unsigned long error_code)
 {
 	/* Empty handler to ignore NMI during early boot */
+	error_putstr("Spurious early NMI ignored.\n");
 }