[nrf fromtree] Bluetooth: Mesh: make blob_io_flash support smaller blocks

Only erase a page when starting the first block on that page.
This fixes a bug where, if the block size is smaller than the page size,
the entire page would be erased upon start of each block when writing,
meaning only the final block on each page would be retained. This works
because blocks are always received in order.

Signed-off-by: Ludvig Jordet <[email protected]>
(cherry picked from commit 99190cb)

Author: ludvigsj

subsys/bluetooth/mesh/blob_io_flash.c

@@ -83,9 +83,13 @@ static inline int erase_device_block(const struct flash_area *fa, off_t start, s
 		return err;
 	}
 
+	if (start != page.start_offset) {
+		/* Only need to erase when starting the first block on the page. */
+		return 0;
+	}
+
 	/* Align to page boundary. */
 	size = page.size * DIV_ROUND_UP(size, page.size);
-	start = page.start_offset;
 
 	return flash_area_erase(fa, start, size);
 }