[JFFS2] Check whether garbage-collection actually obsoleted its victim.

In OLPC trac #4184 we found a case where a corrupted node didn't
actually get obsoleted when we tried to garbage-collect it. So we wrote
out many million copies of it, in repeated attempts to obsolete it,
until the flash became full. Don't Do That.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>