ext4: use BUG_ON instead of if condition followed by BUG

BUG_ON would be better.

This issue was detected with the help of Coccinelle.

Reported-by: Zeal robot <zealci@zte.com.cn>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Link: https://lore.kernel.org/r/20211228073252.580296-1-xu.xin16@zte.com.cn


Signed-off-by: Theodore Ts'o <tytso@mit.edu>