net: coap: Update coap context using payload length
net: coap: Update coap context using payload length

When a file is download, `ctc->current` is increaed by the block
size, which is correct for all the blocks except the last one.

 Once the download is compelted, ctx->current must match
`ctx->total_size`.

Signed-off-by: Efrain Calderon <a766981570b342e88f5fe6435336df8d135300f0@aquarobur.com>

galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr