mount: fix cwd_fd leak on clone error

We should close cwd_fd on error paths, found by Coverity Scan:

*** CID 187162:  Resource leaks  (RESOURCE_LEAK)
/criu/mount.c: 1370 in open_mountpoint()
1364                     */
1365                    pid = clone_noasan(ns_open_mountpoint, CLONE_VFORK | CLONE_VM
1366                                    | CLONE_FILES | CLONE_IO | CLONE_SIGHAND
1367                                    | CLONE_SYSVSEM, &ca);
1368                    if (pid == -1) {
1369                            pr_perror("Can't clone helper process");
>>>     CID 187162:  Resource leaks  (RESOURCE_LEAK)
>>>     Handle variable "cwd_fd" going out of scope leaks the handle.
1370                            return -1;
1371                    }
1372
1373                    errno = 0;
1374                    if (waitpid(pid, &status, __WALL) != pid || !WIFEXITED(status)
1375                                    || WEXITSTATUS(status)) {

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>