fix segmentation fault caused by uninitialized mutex

Segmentation fault was raised while trying to restore a process with
tty. Coredump file says this is caused by uninitialized tty_mutex:
        (gdb) where
        #0  0x00000000004d7270 in atomic_add_return (i=1, v=0x0) at
        include/common/asm/atomic.h:34
        #1  0x00000000004d7398 in mutex_lock (m=0x0) at
        include/common/lock.h:151
        #2  0x00000000004d840c in __pty_open_ptmx_index (index=3, flags=2,
        cb=0x4dce50 <open_pty>, arg=0x11, path=0x5562e0 "ptmx") at
        criu/tty.c:603
        #3  0x00000000004dced8 in pty_create_ptmx_index (dfd=17, index=3,
        flags=2) at criu/tty.c:2384

since init_tty_mutex() is reentrantable, just calling it before
mutex_lock()

Signed-off-by: Deng Guangxing <dengguangxing@huawei.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>

criu/tty.c

@@ -600,6 +600,9 @@ static int __pty_open_ptmx_index(int index, int flags,
 
 	memset(fds, 0xff, sizeof(fds));
 
+	if (init_tty_mutex())
+		return -1;
+
 	mutex_lock(tty_mutex);
 
 	for (i = 0; i < ARRAY_SIZE(fds); i++) {