[PATCH] fix for generic_file_write iov problem

Here is the fix for the problem described in

	http://bugzilla.kernel.org/show_bug.cgi?id=4721

Basically, problem is generic_file_buffered_write() is accessing beyond end
of the iov[] vector after handling the last vector.  If we happen to cross
page boundary, we get a fault.

I think this simple patch is good enough.  If we really don't want to
depend on the "count", then we need pass nr_segs to
filemap_set_next_iovec() and decrement it and check it.
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/filemap.c

@@ -1954,7 +1954,9 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 				if (unlikely(nr_segs > 1)) {
 					filemap_set_next_iovec(&cur_iov,
 							&iov_base, status);
-					buf = cur_iov->iov_base + iov_base;
+					if (count)
+						buf = cur_iov->iov_base +
+							iov_base;
 				} else {
 					iov_base += status;
 				}