net: core: rtnetlink: use BUG_ON instead of if condition followed by BUG

Use BUG_ON instead of if condition followed by BUG in do_setlink.

This issue was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>