arm64: vdso: remove broken, redundant sequence counting for timezones

This patch is an arm64 version of ce73ec6db4 ("powerpc/vdso: Remove
redundant locking in update_vsyscall_tz()").

Timezone data is not protected, so the sequence counter is not required
to ensure consistency. Furthermore, having multiple paths updating the
counter leads to a race between update_vsyscall and update_vsyscall_tz,
so remove the timezone sequence counting from both the kernel and the
vdso.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/vdso.c

@@ -252,10 +252,6 @@ void update_vsyscall(struct timekeeper *tk)
 
 void update_vsyscall_tz(void)
 {
-	++vdso_data->tb_seq_count;
-	smp_wmb();
 	vdso_data->tz_minuteswest	= sys_tz.tz_minuteswest;
 	vdso_data->tz_dsttime		= sys_tz.tz_dsttime;
-	smp_wmb();
-	++vdso_data->tb_seq_count;
 }

arch/arm64/kernel/vdso/gettimeofday.S

@@ -73,8 +73,6 @@ ENTRY(__kernel_gettimeofday)
 	/* If tz is NULL, return 0. */
 	cbz	x1, 3f
 	ldp	w4, w5, [vdso_data, #VDSO_TZ_MINWEST]
-	seqcnt_read w9
-	seqcnt_check w9, 1b
 	stp	w4, w5, [x1, #TZ_MINWEST]
 3:
 	mov	x0, xzr