mount.nfs: Support an "rdma" mount option

mount.nfs: Support an "rdma" mount option

The kernel NFS client's mount option parser recognizes a stand-alone
"rdma" mount option, similar to the legacy "udp" and "tcp" options.

The mount.nfs command text-based mount option parser used to pass
"rdma" straight to the kernel, but since we've started handling MNT in
the kernel instead of in user space, "rdma" on the command line has
not worked.

Until now, no-one has noticed, especially since an "rdma" mount option
isn't documented in nfs(5).

Support "rdma" in mount.nfs command, and document it in nfs(5).

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