ceph: fix potential double free

We re-run the loop but we don't re-set the attrs pointer back to NULL.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Reviewed-by: Alex Elder <elder@inktank.com>

fs/ceph/xattr.c

@@ -457,6 +457,7 @@ start:
 			for (i = 0; i < numattr; i++)
 				kfree(xattrs[i]);
 			kfree(xattrs);
+			xattrs = NULL;
 			goto start;
 		}
 		err = -EIO;