subsys: fs/nvs: fix writes when write_block_size != 1

The current code computes the block-aligned len by ANDing the len with
~write_block_size instead of ~(write_block_size - 1).

In addition the compute value can be 0 (for lengths that are less than
the block size), so the first flash write might have to be skipped.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>