NFSD: Fix callback decoder status codes

NFSD: Fix callback decoder status codes

fs/nfsd/nfs4callback.c implements a callback client. Thus its XDR
decoders are decoding replies, not calls.

NFS4ERR_BAD_XDR is an on-the-wire status code that reports that the
client sent a corrupted RPC /call/. It's not used as the internal
error code when a /reply/ can't be decoded, since that kind of
failure is never reported to the sender of that RPC message.

Instead, a reply decoder should return -EIO, as the reply decoders
in the NFS client do.

Fixes: 6487a13b5c6b ("NFSD: add support for CB_GETATTR callback")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>