fs/nvs: fix initialization hang if sector is full

During initialization NVS analyzes open sector for
find write addresses. It was possible to infinite loop
in case when there was no space for any new ATE in the sector.

This patch introduce check for space available in the sector which
fix the issue.

fixes #17891

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