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

Fix the following coccinelle reports:
./arch/um/kernel/mem.c:77:3-6: WARNING: Use BUG_ON instead of if
condition followed by BUG.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Richard Weinberger <richard@nod.at>