mm: cleanup is_highmem()

It is unnecessary to add CONFIG_HIGHMEM check in is_highmem(), which has
been done in is_highmem_idx(), and move is_highmem() close to
is_highmem_idx().  This has no functional impact.

Link: https://lkml.kernel.org/r/20220726131816.149075-1-wangkefeng.wang@huawei.com
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Kefeng Wang

include/linux/mmzone.h

@@ -1137,15 +1137,6 @@ static inline int is_highmem_idx(enum zone_type idx)
 #endif
 }
 
-#ifdef CONFIG_ZONE_DMA
-bool has_managed_dma(void);
-#else
-static inline bool has_managed_dma(void)
-{
-	return false;
-}
-#endif
-
 /**
  * is_highmem - helper function to quickly check if a struct zone is a
  *              highmem zone or not.  This is an attempt to keep references
@@ -1155,12 +1146,17 @@ static inline bool has_managed_dma(void)
  */
 static inline int is_highmem(struct zone *zone)
 {
-#ifdef CONFIG_HIGHMEM
 	return is_highmem_idx(zone_idx(zone));
+}
+
+#ifdef CONFIG_ZONE_DMA
+bool has_managed_dma(void);
 #else
-	return 0;
-#endif
+static inline bool has_managed_dma(void)
+{
+	return false;
 }
+#endif
 
 /* These two functions are used to setup the per zone pages min values */
 struct ctl_table;