nfsd4: leave reply buffer space for failed setattr

This fixes an ommission from 18032ca0
"NFSD: Server implementation of MAC Labeling", which increased the size
of the setattr error reply without increasing COMPOUND_ERR_SLACK_SPACE.

Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>