Fix a potentially infinite loop in check_overlap()

Fix a potentially infinite loop in check_overlap()

If the following happens:
* check_overlap() finds an overlap.
* All other threads finish after the overlap has been found and before
  the next iteration of the do/while loop starts.

Then the do/while loop in check_overlap() will iterate forever. Fix this
by rewriting check_overlap() such that this cannot happen.

This patch fixes the following Coverity complaint:

CID 184174 (#2 of 2): Double lock (LOCK)

Fixes: c06379a65d46 ("fio: enable overlap checking with offload submission")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>