net: bridge: Extract br_vlan_add_existing()

Extract the code that deals with adding a preexisting VLAN to bridge CPU
port to a separate function. A follow-up patch introduces a need to roll
back operations in this block due to an error, and this split will make
the error-handling code clearer.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>