Bluetooth: Fix skb allocation in mgmt_remote_name() & mgmt_device_connected()

This patch fixes skb allocation, as lack of space for ev might push skb
tail beyond its end.
Also introduce eir_precalc_len() that can be used instead of magic
numbers for similar eir operations on skb.

Fixes: cf1bce1d ("Bluetooth: mgmt: Make use of mgmt_send_event_skb in MGMT_EV_DEVICE_FOUND")
Fixes: e9674143 ("Bluetooth: mgmt: Make use of mgmt_send_event_skb in MGMT_EV_DEVICE_CONNECTED")
Signed-off-by: Angela Czubak <acz@semihalf.com>
Signed-off-by: Marek Maslanka <mm@semihalf.com>
Signed-off-by: Radoslaw Biernacki <rad@semihalf.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>