9p: Use BUG_ON instead of if condition followed by BUG.

This issue was detected with the help of Coccinelle.

Link: https://lkml.kernel.org/r/20211112092547.9153-1-zhang.mingyu@zte.com.cn


Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Zhang Mingyu <zhang.mingyu@zte.com.cn>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>