Btrfs: Avoid using __GFP_HIGHMEM with slab allocator

btrfs_releasepage may call kmem_cache_alloc indirectly,
and provide same GFP flags it gets to kmem_cache_alloc.
So it's possible to use __GFP_HIGHMEM with the slab
allocator.
Signed-off-by: NYan Zheng <zheng.yan@oracle.com>

fs/btrfs/inode.c

@@ -4263,7 +4263,7 @@ static int btrfs_releasepage(struct page *page, gfp_t gfp_flags)
 {
 	if (PageWriteback(page) || PageDirty(page))
 		return 0;
-	return __btrfs_releasepage(page, gfp_flags);
+	return __btrfs_releasepage(page, gfp_flags & GFP_NOFS);
 }
 
 static void btrfs_invalidatepage(struct page *page, unsigned long offset)