Under some circumstances, the CLNT_CALL_MBUF()
in clnt_call_private() needs the correct vnet set.
Since the clnt_call_private() call in rpctls_impl.c
is always for the NFS client, it must always set
vnet0. (NFS client mounts can only be done in
vnet0.)
This patch adds KRPC_CURVNET_SET() and
KRPC_CURVNET_RESTORE() around the
clnt_call_private() call to fix this.