test/ocf: enable serialize_overlap flag for FIO

Set serialize_overlap=1 to prevent data races.
This solves the issue of fio --verify fails in OCF tests
  when persistent metadata is enabled.
  Example:
     https://dqtibwqq6s6ux.cloudfront.net/public_build/autotest-per-patch_33325.html

The serialize_overlap flag prevents from
  creating undefined regions by doing simultaneus writes
  (occurs on certain storages).
  More on that:
     https://fio.readthedocs.io/en/latest/fio_doc.html#cmdoption-arg-serialize-overlap



We should probably use this flag for all FIO --verify tests.

Change-Id: Ia4f73f9900835faf4f94018f1d148fb64f06dc8a
Signed-off-by: Vitaliy Mysak <vitaliy.mysak@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/456991


Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>