fs/nvs: fix delete ate accommodation

NVS always leave space for deletion ate in a full sector.
Even that it was not possible to write delete nvs_ate to the
full sector. Because of that it was possible to fill up NVS,
and be able to delete nothing.

This patch introduce recognition of case
the delete ate is writing, and allow to write such nvs_ate to
opened full sector.

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