mips/kvm: Use BUG_ON instead of if condition followed by BUG

BUG_ON uses unlikely in if(), it can be optimized at compile time.

Usually, the condition in if() is not satisfied. In my opinion,
this can improve the efficiency of the multi-stage pipeline.

Signed-off-by: zhouchuangao <zhouchuangao@vivo.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>