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>

arch/um/kernel/mem.c

@@ -85,8 +85,7 @@ static void __init one_md_table_init(pud_t *pud)
 		      __func__, PAGE_SIZE, PAGE_SIZE);
 
 	set_pud(pud, __pud(_KERNPG_TABLE + (unsigned long) __pa(pmd_table)));
-	if (pmd_table != pmd_offset(pud, 0))
-		BUG();
+	BUG_ON(pmd_table != pmd_offset(pud, 0));
 #endif
 }