verify: header seed check for read only workloads

verify: header seed check for read only workloads

For read jobs, users should have the option to verify header seeds at a
later point of time. Currently for read jobs header seeds are not
generated

Consider the below mentioned write followed by read workloads. Here fio
should allow header seed verification.

fio --name=test --filesize=16k --rw=randwrite --verify=md5
fio --name=test --filesize=16k --rw=randread --verify=md5 --verify_header_seed=1

However there are other scenarios where header seed verification will
fail. These include:
 * randrepeat is set to false, leading to different seed across runs.
 * randseed is different across write and read workloads.
 * Read workload is changed from sequential to random or vice versa
   across runs.
 * Read workloads run in the same invocation as write, i.e. a write job
   followed by a stonewall read job. Header seed verification will fail
   because random seeds vary between jobs. Refer t/jobs/t0029.fio

If verify_header_seed is explicitly enabled, fio will verify header seed
for the workload.

This reverts part of commit mentioned below
Fixes: def41e55 ("verify: decouple seed generation from buffer fill")

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