Btrfs: Only limit btree writeback for pdflush

Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/disk-io.c

@@ -210,7 +210,7 @@ static int btree_writepages(struct address_space *mapping,
 {
 	struct extent_map_tree *tree;
 	tree = &BTRFS_I(mapping->host)->extent_tree;
-	if (wbc->sync_mode == WB_SYNC_NONE) {
+	if (wbc->sync_mode == WB_SYNC_NONE && current_is_pdflush()) {
 		u64 num_dirty;
 		u64 start = 0;
 		unsigned long thresh = 96 * 1024 * 1024;