NFSD: Replace the internals of the READ_BUF() macro

NFSD: Replace the internals of the READ_BUF() macro

[ Upstream commit c1346a1216ab5cb04a265380ac9035d91b16b6d5 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>