storage/stream: fix possible unaligned write on buffer flush

On buffer flush request it is very probably that write buffer
contains amount of data which is non write-block-size aligned.
Flash memory need to be write at minimal by write-block-size chunks.
This patch addresses mechanism which ensure such behavior by adding
missing bytes.

fixes #25471

streamer buffer size should be multiple write-block-size of
the flash device in order to avoid unaligned flash write
request.

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