osd: Do not sent PDWs if read count > k

osd: Do not sent PDWs if read count > k

The main point of PDW (as currently implemented) is to reduce the amount
of reading performed by the primary when preparing for a read-modify-write (RMW).

It was making the assumption that if any recovery was required by a
conventional RMW, then a PDW is always better. This was an incorrect assumption
as a conventional RMW performs at most K reads for any plugin which
supports PDW. As such, we tweak this logic to perform a conventional RMW
if the PDW is going to read k or more shards.

This should improve performance in some minor areas.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>
(cherry picked from commit cffd10f3cc82e0aef29209e6e823b92bdb0291ce)