um: Replace if (cond) BUG() with BUG_ON()

Fix the following coccinelle reports:

./arch/um/kernel/mem.c:89:2-5: WARNING: Use BUG_ON instead of if
condition followed by BUG.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Signed-off-by: Richard Weinberger <richard@nod.at>