add ioflag and cred arguments to VOP_ALLOCATE() for the nfs server

[rmacklem]

Summary

When the NFSv4.2 server does a VOP_ALLOCATE(), it needs
the operation to be done for the RPC's credential and not
td_ucred. It also needs the writing to be done synchronously.

This patch adds "ioflag" and "cred" arguments to VOP_ALLOCATE()
and modifies vop_stdallocate() to use these arguments.

Test Plan

Tested with an NFSv4.2 client for a non-root user.

[khng]

I forgot to mention the corresponding VOP_ALLOCATE.9 has to be updated as well.