net: lwm2m: Fix blockwise Ack NUM calculation

After the PR #85000 the calculation of NUM field of
Block 1 option on CoAP Ack packet started to advance
to next packet block.

We should not update the ctx->current field because it is
used for calculating the NUM field in response packet.
It should point to beginning of the payload,
so the response is correct.

Leshan server don't seem to care about this, but Coiote does.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>