powerpc/vdso32: Fallback on getres syscall when clock is unknown

There are other clocks than the standard ones, for instance
per process clocks. Therefore, being above the last standard clock
doesn't mean it is a bad clock. So, fallback to syscall instead
of returning -EINVAL inconditionaly.

Fixes: e33ffc95

 ("powerpc/vdso32: implement clock_getres entirely")
Cc: stable@vger.kernel.org # v5.6+
Reported-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>
Link: https://lore.kernel.org/r/7316a9e2c0c2517923eb4b0411c4a08d15e675a4.1589017281.git.christophe.leroy@csgroup.eu