Allow random overwrite workloads to write io_limit

Allow random overwrite workloads to write io_limit

Limiting the write phase of (random) overwrite workloads to 'size'
before the verify phase doesn't make sense, so allow them to write
up to io_limit.  This better matches verify_only behavior, and
allows the [verify-job] below to succeed:

[global]
thread=1
direct=1
ioengine=sync
verify=meta
bs=1k
offset=0
size=5M
io_limit=6M
norandommap=1
verify_fatal=1
randseed=42

[write-job]
filename=datafile.tmp
rw=randwrite
do_verify=0

[verify-job]
stonewall
create_serialize=0
filename=datafile.tmp
rw=randwrite
do_verify=1
verify_only=1

Signed-off-by: Justin Eno <jeno@micron.com>
Signed-off-by: Jens Axboe <axboe@fb.com>