x86/boot: GDT limit value should be size - 1

The limit value for the GDTR should be such that adding it to the base
address gives the address of the last byte of the GDT, i.e. it should be
one less than the size, not the size.
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200202171353.3736319-7-nivedita@alum.mit.eduSigned-off-by: Ard Biesheuvel <ardb@kernel.org>

arch/x86/boot/compressed/head_64.S

@@ -624,12 +624,12 @@ SYM_FUNC_END(.Lno_longmode)
 
 	.data
 SYM_DATA_START_LOCAL(gdt64)
-	.word	gdt_end - gdt
+	.word	gdt_end - gdt - 1
 	.quad   0
 SYM_DATA_END(gdt64)
 	.balign	8
 SYM_DATA_START_LOCAL(gdt)
-	.word	gdt_end - gdt
+	.word	gdt_end - gdt - 1
 	.long	gdt
 	.word	0
 	.quad	0x00cf9a000000ffff	/* __KERNEL32_CS */