net: rbridge: fix bridge name copying

directly copy data in rbr->name instead of using a local variable
also fix an unsafe strncat with size IFNAMSIZ-4; sizeof _rbr is 5 and
not 4.

Signed-off-by: William Dauchy <william@gandi.net>
Signed-off-by: Ahmed Amamou <ahmed@gandi.net>
net: rbridge: remove unused bridge name

Ahmed finally agreed that since we are not using rbr->name and
rbr->rbr_bridgename, we can get rid of them for now.
Might be useful in the future for other functionalities.

Signed-off-by: William Dauchy <william@gandi.net>
Signed-off-by: Ahmed Amamou <ahmed@gandi.net>