mips: vdso: Fix __arch_get_hw_counter()

On some MIPS variants (e.g. MIPS r1), vDSO clock_mode is set to
VDSO_CLOCK_NONE.

When VDSO_CLOCK_NONE is set the expected kernel behavior is to fallback
on syscalls. To do that the generic vDSO library expects UULONG_MAX as
return value of __arch_get_hw_counter().

Fix __arch_get_hw_counter() on MIPS defining a __VDSO_USE_SYSCALL case
that addressed the described scenario.
Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Tested-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: linux-mips@vger.kernel.org

arch/mips/include/asm/vdso/gettimeofday.h

@@ -24,6 +24,8 @@
 
 #define VDSO_HAS_CLOCK_GETRES		1
 
+#define __VDSO_USE_SYSCALL		ULLONG_MAX
+
 #ifdef CONFIG_MIPS_CLOCK_VSYSCALL
 
 static __always_inline long gettimeofday_fallback(
@@ -205,7 +207,7 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode)
 		break;
 #endif
 	default:
-		cycle_now = 0;
+		cycle_now = __VDSO_USE_SYSCALL;
 		break;
 	}