NFS: Introduce new-style XDR encoding functions for NFSv2

We're interested in taking advantage of the safety benefits of
xdr_streams.  These data structures allow more careful checking for
buffer overflow while encoding.  More careful type checking is also
introduced in the new functions.

For efficiency, we also eventually want to be able to pass xdr_streams
from call_encode() to all XDR encoding functions, rather than building
an xdr_stream in every XDR encoding function in the kernel.  To do
this means all encoders must be ready to handle a passed-in
xdr_stream.

The new encoders follow the modern paradigm for XDR encoders: BUG on
any error, and always return a zero status code.

Static helper functions are left without the "inline" directive.  This
allows the compiler to choose automatically how to optimize these for
size or speed.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>