vfs: Add "ioflag" and "cred" arguments to VOP_ALLOCATE

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.

The VOP_ALLOCATE.9 man page will be patched separately.

Reviewed by: khng, kib
Differential Revision: https://reviews.freebsd.org/D32865