ignore EINVAL error for ftruncate on /dev/zero

git-svn-id: svn://svn.lighttpd.net/xcache/trunk@768 c26eb9a1-5813-0410-bd6c-c2e55f420ca7

mmap.c

@@ -187,7 +187,7 @@ static XC_SHM_INIT(xc_mmap_init) /* {{{ */
 		}
 	}
 
-	if (ftruncate(fd, size) != 0) {
+	if (ftruncate(fd, size) != 0 && errno != EINVAL) {
 		perror(shm->name);
 		errstr = "Failed to ftruncate the file";
 		goto err;