[JFFS2] Add paranoia debugging for superblock counts

The problem fixed in commit 014b164e
(space leak with in-band cleanmarkers) would have been caught a lot
quicker if our paranoid debugging mode had included adding up the size
counts from all the eraseblocks and comparing the totals with the counts
in the superblock. Add that.

Make jffs2_mark_erased_block() file the newly-erased block on the
free_list before calling the debug function, to make it happy.

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