nfsd4: buffer-length check for SUPPATTR_EXCLCREAT

This was an omission from 8c18f205
"nfsd41: SUPPATTR_EXCLCREAT attribute".

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

(cherry picked from commit de3997a7)
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

fs/nfsd/nfs4xdr.c

@@ -2416,6 +2416,8 @@ nfsd4_encode_fattr(struct svc_fh *fhp, struct svc_export *exp,
 		WRITE64(stat.ino);
 	}
 	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);