btrfs: fix wrong allocating flag when reading page

the space cache use extent_readpages() to read free space information,
so we can not use GFP_KERNEL flag to allocate memory, or it may lead
to deadlock.
Signed-off-by: Itaru Kitayama <kitayama@cl.bb4u.ne.jp>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.c

@@ -2681,7 +2681,7 @@ int extent_readpages(struct extent_io_tree *tree,
 		prefetchw(&page->flags);
 		list_del(&page->lru);
 		if (!add_to_page_cache_lru(page, mapping,
-					page->index, GFP_KERNEL)) {
+					page->index, GFP_NOFS)) {
 			__extent_read_full_page(tree, page, get_extent,
 						&bio, 0, &bio_flags);
 		}