NFS: Add separate mountd status code decoders for each mountd version

Introduce data structures and xdr_stream-based decoding functions for
unmarshalling mountd status codes properly.

Mountd version 3 uses specific standard error return codes that are
not errno values and not NFS3ERR_ values.  These have a well-defined
standard mapping to local errno values.  Introduce data structures
and a decoder function that map these status codes to local errno
values properly.  This is new functionality (but not used yet).

Version 1 mountd status values are defined by RFC 1094 as UNIX error
values (errno values).  Errno values on heterogeneous systems do not
necessarily match each other.  To avoid exposing possibly incorrect
errno values to upper layers, the current XDR decoder converts all
non-zero MNT version 1 status codes to -EACCES.

The OpenGroup XNFS standard provides a mapping similar to but smaller
than the version 3 error codes.  Implement a decoder that uses the XNFS
error codes, replacing the current decoder.

For both mountd protocol versions, map unrecognized errors to -EACCES.

Finally we introduce a replacement data structure for mnt_fhstatus
at this time, which is used by the new XDR decoders.  In addition to
documenting that the status value returned by the XDR decoders is
always an errno, this new structure will be expanded in subsequent
patches.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>