Bluetooth: Mesh: Verify network buffer max len

As network_decode needs a target buffer for decoding, the max PDU length
must be checked to prevent overflow on the target buffer. When receiving
a proxy configuration message with excessive length, there's no previous
check for this.

Also pulls the NET PDU length defines out into net.h, so they can be
used when defining the target buffers.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>