fs/nvs: fix startup for 2-sectors configuration

This patch fixes following bug:

After first GC operation the 1st sector had become scratch
and the 2nd sector had became write sector. After that NVS
was initialize (via reboot) again - it recognized the 1st
sector as write sector and 2nd as undone GC destination sector,
therefore it cleared 2nd sector and  re-run GC, which implied data loss.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>