zms: optimize write function by skipping unnecessary reads

when performing a write ZMS checks if the data exists in the storage to
avoid double writing the same data and save some memory cycle life time.
However this downgrades the write performance.
Enable this feature only when CONFIG_ZMS_NO_DOUBLE_WRITE is enabled.

Signed-off-by: Riadh Ghaddab <rghaddab@baylibre.com>