NFSD: fix decoding in nfs4_xdr_dec_cb_getattr

If a client were to send an error to a CB_GETATTR call, the code
erronously continues to try decode past the error code. It ends
up returning BAD_XDR error to the rpc layer and then in turn
trigger a WARN_ONCE in nfsd4_cb_done() function.

Fixes: 6487a13b ("NFSD: add support for CB_GETATTR callback")
Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>