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>

arch/um/kernel/mem.c

@@ -73,8 +73,7 @@ static void __init one_page_table_init(pmd_t *pmd)
 
 		set_pmd(pmd, __pmd(_KERNPG_TABLE +
 					   (unsigned long) __pa(pte)));
-		if (pte != pte_offset_kernel(pmd, 0))
-			BUG();
+		BUG_ON(pte != pte_offset_kernel(pmd, 0));
 	}
 }