Btrfs: don't bug on enomem in readpage

Get rid of the BUG_ON(ret == -ENOMEM) in __extent_read_full_page.  Thanks,
Reported-by: NJérôme Poulin <jeromepoulin@gmail.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>

fs/btrfs/extent_io.c

@@ -2751,12 +2751,15 @@ static int __extent_read_full_page(struct extent_io_tree *tree,
 					 end_bio_extent_readpage, mirror_num,
 					 *bio_flags,
 					 this_bio_flag);
-			BUG_ON(ret == -ENOMEM);
+			if (!ret) {
 				nr++;
 				*bio_flags = this_bio_flag;
 			}
-		if (ret)
+		}
+		if (ret) {
 			SetPageError(page);
+			unlock_extent(tree, cur, cur + iosize - 1);
+		}
 		cur = cur + iosize;
 		pg_offset += iosize;
 	}