bridge: vlan tunnel id info range fill size calc cleanups

This fixes a bug and cleans up tunnelid range size
calculation code by using consistent variable names
and checks in size calculation and fill functions.

tested for a few cases of vlan-vni range mappings:
(output from patched iproute2):
$bridge vlan showtunnel
port     vid        tunid
vxlan0   100-105    1000-1005
         200        2000
         210        2100
         211-213    2100-2102
         214        2104
         216-217    2108-2109
         219        2119

Fixes: efa5356b ("bridge: per vlan dst_metadata netlink support")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>