mount.nfs: Prepare way for "vers=4,rdma" mounts

At some point, when the kernel starts to support "vers=4,rdma" mounts,
we will want the mount.nfs command to pass "vers=4,rdma" mounts
instead of rejecting them.

Assuming that the kernel will reject these today with EPROTONOSUPPORT,
that would cause the version fallback logic to go to "vers=3,rdma"
automatically.  So the extra check we have now is not needed anyway.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>