[JFFS2] Fix memory leak in scan code; improve comments.

If we had to allocate extra space for the summary node, we weren't
correctly freeing it when jffs2_sum_scan_sumnode() returned nonzero --
which is both the success and the failure case. Only when it returned
zero, which means fall through to the full scan, were we correctly freeing
the buffer.

Document the meaning of those return codes while we're at it.

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