jffs2: leaking jffs2_summary in function jffs2_scan_medium

In case of an error returned by file_dirty() 's' is not freed as the cleanup
path is skipped.

Reported by Coverity.

Signed-off-by: Christian Engelmayer <christian.engelmayer@frequentis.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>