Use BUG_ON instead of if condition followed by BUG.

Something to notice in this particular case is that unlikely()
is already being called inside BUG_ON macro.

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>