NFSD: Replace the internals of the READ_BUF() macro

Convert the READ_BUF macro in nfs4xdr.c from open code to instead
use the new xdr_stream-style decoders already in use by the encode
side (and by the in-kernel NFS client implementation). Once this
conversion is done, each individual NFSv4 argument decoder can be
independently cleaned up to replace these macros with C code.

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