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: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfsd.h

@@ -282,7 +282,7 @@ void		nfsd_lockd_shutdown(void);
  * reason.
  */
 #define	COMPOUND_SLACK_SPACE		140    /* OP_GETFH */
-#define COMPOUND_ERR_SLACK_SPACE	12     /* OP_SETATTR */
+#define COMPOUND_ERR_SLACK_SPACE	16     /* OP_SETATTR */
 
 #define NFSD_LAUNDROMAT_MINTIMEOUT      1   /* seconds */