nfscl: Add support for read delegations and atomic upgrade

For NFSv4.1/4.2, an atomic upgrade of a delegation from a
read delegation to a write delegation is allowed and can
result in significantly improved performance.
This patch adds this upgrade to the NFSv4.1/4.2 client and
enables use of read delegations.

For a test case of building a FreeBSD kernel (sources and
output objects) over a NFSv4.2 mount, these changes reduced
the elapsed time by 30% and included a reduction of 80% for
RPC counts when delegations were enabled. As such, with this
patch there are at least certain cases where enabling
delegations seems to be worth the increased complexity they
bring.

This patch should only affect the NFSv4.1/4.2 behaviour
when delegations are enabled, which is not the default.

MFC after: 1 month