[JFFS2][SUMMARY] Fix a summary collecting bug.

In some special case (padding because of sync
or umount) it can be possible that summary
information is not fit to the end of the erase
block. In these cases the collecting of summary
is disabled for this erase block.

The problem was that this was not respected
by jffs2_sum_add_kvec(). This patch fix this
bug.

From: Zoltan Sogor <weth@inf.u-szeged.hu>
Signed-off-by: Ferenc Havasi <havasi@inf.u-szeged.hu>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>