Fix truncate restart error

Fix for a rare problem seen under stress in data=journal mode: if we
have to restart a truncate transaction while traversing the inode's
direct blocks, we need to deal with bh==NULL in ext3_clear_blocks.

--- linux-2.4.19-ext3/fs/ext3/inode.c.=K0009=.orig	Fri Oct 11 15:52:00 2002
+++ linux-2.4.19-ext3/fs/ext3/inode.c	Fri Oct 11 15:52:01 2002
@@ -1591,8 +1591,10 @@
 		}
 		ext3_mark_inode_dirty(handle, inode);
 		ext3_journal_test_restart(handle, inode);
-		BUFFER_TRACE(bh, "get_write_access");
-		ext3_journal_get_write_access(handle, bh);
+		if (bh) {
+			BUFFER_TRACE(bh, "retaking write access");
+			ext3_journal_get_write_access(handle, bh);
+		}
 	}
 
 	/*