fs: nvs: fail on >= 256 B block sizes
fs: nvs: fail on >= 256 B block sizes

The current code truncates the size_t block size to a u8_t, which
causes a 256 or 512 byte block size to be truncated to zero and
passing the size check.

Check as a size_t then truncate once the check passes.

Signed-off-by: Michael Hope <c71d7810e3c04e4d6ad9df326b82536b86a1bfbc@google.com>

zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr