Bluetooth: Mesh: Fix relaying conditions for Adv & GATT bearers

The bt_mesh_net_relay() function was missing several important checks
for whether a PDU should be relayed or not. In particular, it would
relay a packet from adv to adv even if the Relay state was set to
disabled, as long as GATT Proxy was set to enabled. The code would
also relay packets to the GATT Proxy bearer if the Relay state was set
to enabled but GATT Proxy was set to disabled. This patch addresses
both of these issues.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>