[JFFS2] Disable summary after wbuf recovery

After a write error, any data in the write buffer must
be relocated.  This is handled by the jffs2_wbuf_recover
function.  This function does not fix up the erase block
summary information that is collected for writing at the
end of the block, which results in an incorrect summary
(or BUG if the summary was found to be empty).

As the summary is not essential (it is an optimisation),
it may be disabled for the current erase block when this
situation arises.  This patch does that.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>