NFS: add "[no]resvport" mount option

NFS: add "[no]resvport" mount option

The standard default security setting for NFS is AUTH_SYS.  An NFS
client connects to NFS servers via a privileged source port and a
fixed standard destination port (2049).  The client sends raw uid and
gid numbers to identify users making NFS requests, and the server
assumes an appropriate authority on the client has vetted these
values because the source port is privileged.

On Linux, by default in-kernel RPC services use a privileged port in
the range between 650 and 1023 to avoid using source ports of well-
known IP services.  Using such a small range limits the number of NFS
mount points and the number of unique NFS servers to which a client
can connect concurrently.

An NFS client can use unprivileged source ports to expand the range of
source port numbers, allowing more concurrent server connections and
more NFS mount points.  Servers must explicitly allow NFS connections
from unprivileged ports for this to work.

In the past, bumping the value of the sunrpc.max_resvport sysctl on
the client would permit the NFS client to use unprivileged ports.
Bumping this setting also changes the maximum port number used by
other in-kernel RPC services, some of which still required a port
number less than 1023.

This is exacerbated by the way source port numbers are chosen by the
Linux RPC client, which starts at the top of the range and works
downwards.  It means that bumping the maximum means all RPC services
requesting a source port will likely get an unprivileged port instead
of a privileged one.

Changing this setting effects all NFS mount points on a client.  A
sysadmin could not selectively choose which mount points would use
non-privileged ports and which could not.

Lastly, this mechanism of expanding the limit on the number of NFS
mount points was entirely undocumented.

To address the need for the NFS client to use a large range of source
ports without interfering with the activity of other in-kernel RPC
services, we introduce a new NFS mount option.  This option explicitly
tells only the NFS client to use a non-privileged source port when
communicating with the NFS server for one specific mount point.

This new mount option is called "resvport," like the similar NFS mount
option on FreeBSD and Mac OS X.  A sister patch for nfs-utils will be
submitted that documents this new option in nfs(5).

The default setting for this new mount option requires the NFS client
to use a privileged port, as before.  Explicitly specifying the
"noresvport" mount option allows the NFS client to use an unprivileged
source port for this mount point when connecting to the NFS server
port.

This mount option is supported only for text-based NFS mounts.

[ Sidebar: it is widely known that security mechanisms based on the
  use of privileged source ports are ineffective.  However, the NFS
  client can combine the use of unprivileged ports with the use of
  secure authentication mechanisms, such as Kerberos.  This allows a
  large number of connections and mount points while ensuring a useful
  level of security.

  Eventually we may change the default setting for this option
  depending on the security flavor used for the mount.  For example,
  if the mount is using only AUTH_SYS, then the default setting will
  be "resvport;" if the mount is using a strong security flavor such
  as krb5, the default setting will be "noresvport." ]

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
[Trond.Myklebust@netapp.com: Fixed a bug whereby nfs4_init_client()
was being called with incorrect arguments.]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>