nfsd: Fix delegation handled for 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 signoficantly improved performance.

This patch adds support for this atomic upgrade, plus fixes
a couple of other delegation related bugs. Since there were
three cases where delegations were being issued, the patch
factors this out into a separate function called
nfsrv_issuedelegations().

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

MFC after: 1 month