x86/asm: Add DB flag to 32-bit percpu GDT entry

The D/B size flag for the 32-bit percpu GDT entry was not set.

The Intel manual (vol 3, section 3.4.5) only specifies the meaning of
this flag for three cases:

 1) code segments used for %cs -- doesn't apply here

 2) stack segments used for %ss -- doesn't apply

 3) expand-down data segments -- but we don't have the expand-down flag
    set, so it also doesn't apply here

The flag likely doesn't do anything here, although the manual does also
say: "This flag should always be set to 1 for 32-bit code and data
segments [...]" so we should probably do it anyway.
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20231219151200.2878271-6-vegard.nossum@oracle.com

arch/x86/kernel/setup_percpu.c

@@ -106,7 +106,7 @@ void __init pcpu_populate_pte(unsigned long addr)
 static inline void setup_percpu_segment(int cpu)
 {
 #ifdef CONFIG_X86_32
-	struct desc_struct d = GDT_ENTRY_INIT(DESC_DATA32 & ~_DESC_DB,
+	struct desc_struct d = GDT_ENTRY_INIT(DESC_DATA32,
 					      per_cpu_offset(cpu), 0xFFFFF);
 
 	write_gdt_entry(get_cpu_gdt_rw(cpu), GDT_ENTRY_PERCPU, &d, DESCTYPE_S);