Fix bug with rw sequence offset and io_limit

If you do:

dd if=/dev/zero of=/dev/shm/1M bs=1M count=1
fio --bs=4k --rw=write:4k --filename=/dev/shm/1M --name=go --io_limit=2M

then fio will exit prematurely after having done 128 4KB ios,
so only 512KB are read. This is because after 128 ios, we have
reache the end of the file. Handle this case and wrap around
properly.

Signed-off-by: Jens Axboe <axboe@fb.com>
Fix bug with zone and zone skipping and io_limit

If you do:

dd if=/dev/zero of=/dev/shm/1M bs=1M count=1
fio --bs=4k --rw=write --filename=/dev/shm/1M --name=go \
--zoneskip=4k --zonesize=4k --io_limit=2M

then fio will exit after having performed 512KB of IO, instead
of the specified IO limit. This is a similar issue as was fixed
by commit ac002339c382 with rw sequencing - if we reach the
end of the file with a zone skip, wrap to the beginning.

Signed-off-by: Jens Axboe <axboe@fb.com>
Clarify what number_ios does in the HOWTO/man page/help

Signed-off-by: Jens Axboe <axboe@fb.com>
Fix typo in bs_is_seq_ran option help

Signed-off-by: Jens Axboe <axboe@fb.com>