Modify nfsrv_pnfscreate(), factoring out the part that does the Open RPC
against the DS server to create the DS data file, so that it can be done
by a separate kernel process.
Put all the arguments to this factored out function (nfsrv_dscreate()) in
a structure, so that it can be passed through kproc_create().
This allows creation of the data file on mirrors to be done concurrently.
In future, it might be better to avoid the kproc_create() call for every
file create and have a set of these kernel processes waiting to do the RPCs.