bridge: Fix double-free in br_add_if.

There is a potential double-kfree in net/bridge/br_if.c.  If br_fdb_insert
fails, then the kobject is put back (which calls kfree due to the kobject
release), and then kfree is called again on the net_bridge_port.  This
patch fixes the crash.

Thanks to Stephen Hemminger for the one-line fix.

Signed-off-by: Jeff Hansen <x@jeffhansen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>