Allow 'norandommap' with verify

Allow 'norandommap' with verify

We used to turn off verify completely if norandommap was given, since
fio does not track block rewrites. But we can easily track rewrites,
as long as we don't have to track various extent sizes. So relax
the restriction to only cover norandommap with differing block sizes
and adapt log_io_piece() to always use rbtree inserts for cases where
we do need to check for potential overwrites.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>