nfsd4: allow space for final error return

This post-encoding check should be taking into account the need to
encode at least an out-of-space error to the following op (if any).

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

diff --cc fs/nfsd/nfs4xdr.c
Simple merge