verify: fix verify issue with offest modifiers

verify: fix verify issue with offest modifiers

Offset modifiers such as rw=readwrite:8 or rw=write:4K can create
overlaps. For these cases use RB tree instead of list to log the I/O
entries.

Add a helper function fio_offset_overlap_risk() to decide whether to log
the I/O entry in an RB tree or a list.

Disable header seed verification if there are offset modifiers, unless
its explicitly enabled.

fixes #1503

Signed-off-by: Ankit Kumar <ankit.kumar@samsung.com>