nfsd4: buffer-length check for SUPPATTR_EXCLCREAT

This was an omission from 8c18f2052e756e7d5dea712fc6e7ed70c00e8a39
"nfsd41: SUPPATTR_EXCLCREAT attribute".

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

fs/nfsd/nfs4xdr.c

@@ -2483,6 +2483,8 @@ out_acl:
 			goto out;
 	}
 	if (bmval2 & FATTR4_WORD2_SUPPATTR_EXCLCREAT) {
+		if ((buflen -= 16) < 0)
+			goto out_resource;
 		WRITE32(3);
 		WRITE32(NFSD_SUPPATTR_EXCLCREAT_WORD0);
 		WRITE32(NFSD_SUPPATTR_EXCLCREAT_WORD1);