nfscl: Move delegation high water variables into clientID

Without this patch, the variables used to maintain a high
water limit for delegations are global and apply to all
mounts. This patch moves them into the clientID structure,
which makes them per mount. This is needed to add support
for the CB_RECALL_ANY callback in a future commit.

The only effect of this patch is an increase in the
total number of delegations held if there are multiple NFSv4
mounts to NFSv4 servers with delegations enabled.
Since the default of NFSCLDELEGHIGHWATER is fairly small,
this should not have a significant impact.

MFC after: 2 weeks