pvsync2: Add hipri_percentage option

pvsync2: Add hipri_percentage option

Allow the user to set the probability of a pvsync2 IO being high
priority. Add a new option for the pvsync2 engine called
--hipri_percentage will takes values in the range (0, 100) and use
that as a probability for setting RWF_HIPRI. The default for this
option is 100 so exisiting users of --hirpi are unaffected.

Changes since v3:
  Remove unwanted #ifdefs

Changes since v2:
  s/_probability/_percentage/

Changes since v1:
  Use --hipri_probability rather than impacting --hipri
  Correct the random number generator call

Signed-off-by: Stephen Bates <sbates@raithlin.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>