Add a function nfsm_set() to initialize "struct nfsrv_descript" for building

mbuf lists.

This function is currently trivial, but will that will change when
support for building NFS messages in ext_pgs mbufs is added.
Adding support for ext_pgs mbufs is needed for KERN_TLS, which will
be used to implement nfs-over-tls.

sys/fs/nfs/nfs_commonsubs.c

@@ -4778,3 +4778,14 @@ nfsv4_findmirror(struct nfsmount *nmp)
 	return (ds);
 }
 
+/*
+ * Fill in the fields of "struct nfsrv_descript".
+ */
+void
+nfsm_set(struct nfsrv_descript *nd, u_int offs)
+{
+	struct mbuf *m;
+
+	m = nd->nd_mb;
+	nd->nd_bpos = mtod(m, char *) + offs;
+}

sys/fs/nfs/nfs_var.h

@@ -360,6 +360,7 @@ int nfsv4_sequencelookup(struct nfsmount *, struct nfsclsession *, int *,
 void nfsv4_freeslot(struct nfsclsession *, int);
 struct ucred *nfsrv_getgrpscred(struct ucred *);
 struct nfsdevice *nfsv4_findmirror(struct nfsmount *);
+void nfsm_set(struct nfsrv_descript *, u_int);
 
 /* nfs_clcomsubs.c */
 void nfsm_uiombuf(struct nfsrv_descript *, struct uio *, int);

sys/fs/nfsserver/nfs_nfsdport.c

@@ -5080,7 +5080,7 @@ nfsrv_writedsdorpc(struct nfsmount *nmp, fhandle_t *fhp, off_t off, int len,
 	while (m->m_next != NULL)
 		m = m->m_next;
 	nd->nd_mb = m;
-	nd->nd_bpos = mtod(m, char *) + m->m_len;
+	nfsm_set(nd, m->m_len);
 	NFSD_DEBUG(4, "nfsrv_writedsdorpc: lastmb len=%d\n", m->m_len);
 
 	/* Do a Getattr for the attributes that change upon writing. */