Unlock and release page before returning error.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
---

--- a/mm/filemap.c	Mon Sep  9 15:51:28 2013
+++ b/mm/filemap.c	Mon Sep  9 15:52:54 2013
@@ -1844,6 +1844,7 @@ retry:
 	}
 	err = filler(data, page);
 	if (err < 0) {
+		unlock_page(page);
 		page_cache_release(page);
 		return ERR_PTR(err);
 	}
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