[nrf fromlist] bluetooth: mesh: default BT_BUF_EVT_DISCARDABLE_SIZE to 58

Extended advertising reports as of
zephyrproject-rtos/zephyr#106374 use the
discardable event pool so fragmented chains can be dropped without. That
raised the default discardable buffer size to BT_BUF_EVT_RX_SIZE to fit
general ext-adv reassembly.

Mesh does not need that headroom. The advertising bearer uses
ADV_NONCONN_IND PDUs (Mesh Protocol v1.1 3.3.1), which are legacy
primary-channel advertisements, not fragmented extended advertising
chains. Mesh scan only accepts BT_GAP_ADV_TYPE_ADV_NONCONN_IND, and
legacy reports fit in 58 bytes, the minimum when BT_EXT_ADV is enabled.

Default to 58 for BT_MESH to save RAM.

Upstream PR #: 110080

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>

Author: HaavardRei

subsys/bluetooth/host/Kconfig

@@ -201,6 +201,7 @@ config BT_BUF_EVT_DISCARDABLE_SIZE
 	default $(UINT8_MAX) if BT_CLASSIC
 	# Le Advertising Report event
 	default 43 if !BT_EXT_ADV
+	default 58 if BT_EXT_ADV && BT_MESH
 	default BT_BUF_EVT_RX_SIZE if BT_EXT_ADV
 	help
 	  Maximum support discardable HCI event size of buffers in the separate