nfscl: Move layout high water variables into clientID

Commit f5aff1871d32 moved the delegation high water
variables into the clientID structure, so that they are now
per mount instead of global. This patch does the
same for the layout highwater variables. It happens
that the layout highwater variables are not actually
used. This patch changes the code to use them.
This is needed to add support
for the CB_RECALL_ANY callback in a future commit.

This patch only affects NFSv4.1/4.2 mounts with the "pnfs"
mount option. The effect on these mounts will be minimal,
since layouts are returned when they are stale and this
normally ensures that the highwater mark is never hit.

MFC after: 2 weeks