pstore/ram: (really) fix undefined usage of rounddown_pow_of_two

Previous attempt to fix was b042e474

Suggested use of is_power_of_2() was bogus because is_power_of_2(0) is
false (documented behaviour).

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>