tun: bail out from tun_get_user() if the skb is empty

KMSAN (https://github.com/google/kmsan) reported accessing uninitialized
skb->data[0] in the case the skb is empty (i.e. skb->len is 0):

================================================
BUG: KMSAN: use of uninitialized memory in tun_get_user+0x19ba/0x3770
CPU: 0 PID: 3051 Comm: probe Not tainted 4.13.0+ #3140
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
...
 __msan_warning_32+0x66/0xb0 mm/kmsan/kmsan_instr.c:477
 tun_get_user+0x19ba/0x3770 drivers/net/tun.c:1301
 tun_chr_write_iter+0x19f/0x300 drivers/net/tun.c:1365
 call_write_iter ./include/linux/fs.h:1743
 new_sync_write fs/read_write.c:457
 __vfs_write+0x6c3/0x7f0 fs/read_write.c:470
 vfs_write+0x3e4/0x770 fs/read_write.c:518
 SYSC_write+0x12f/0x2b0 fs/read_write.c:565
 SyS_write+0x55/0x80 fs/read_write.c:557
 do_syscall_64+0x242/0x330 arch/x86/entry/common.c:284
 entry_SYSCALL64_slow_path+0x25/0x25 arch/x86/entry/entry_64.S:245
...
origin:
...
 kmsan_poison_shadow+0x6e/0xc0 mm/kmsan/kmsan.c:211
 slab_alloc_node mm/slub.c:2732
 __kmalloc_node_track_caller+0x351/0x370 mm/slub.c:4351
 __kmalloc_reserve net/core/skbuff.c:138
 __alloc_skb+0x26a/0x810 net/core/skbuff.c:231
 alloc_skb ./include/linux/skbuff.h:903
 alloc_skb_with_frags+0x1d7/0xc80 net/core/skbuff.c:4756
 sock_alloc_send_pskb+0xabf/0xfe0 net/core/sock.c:2037
 tun_alloc_skb drivers/net/tun.c:1144
 tun_get_user+0x9a8/0x3770 drivers/net/tun.c:1274
 tun_chr_write_iter+0x19f/0x300 drivers/net/tun.c:1365
 call_write_iter ./include/linux/fs.h:1743
 new_sync_write fs/read_write.c:457
 __vfs_write+0x6c3/0x7f0 fs/read_write.c:470
 vfs_write+0x3e4/0x770 fs/read_write.c:518
 SYSC_write+0x12f/0x2b0 fs/read_write.c:565
 SyS_write+0x55/0x80 fs/read_write.c:557
 do_syscall_64+0x242/0x330 arch/x86/entry/common.c:284
 return_from_SYSCALL_64+0x0/0x6a arch/x86/entry/entry_64.S:245
================================================

Make sure tun_get_user() doesn't touch skb->data[0] unless there is
actual data.

C reproducer below:
==========================
    // autogenerated by syzkaller (http://github.com/google/syzkaller

)

    #define _GNU_SOURCE

    #include <fcntl.h>
    #include <linux/if_tun.h>
    #include <netinet/ip.h>
    #include <net/if.h>
    #include <string.h>
    #include <sys/ioctl.h>

    int main()
    {
      int sock = socket(PF_INET, SOCK_STREAM, IPPROTO_IP);
      int tun_fd = open("/dev/net/tun", O_RDWR);
      struct ifreq req;
      memset(&req, 0, sizeof(struct ifreq));
      strcpy((char*)&req.ifr_name, "gre0");
      req.ifr_flags = IFF_UP | IFF_MULTICAST;
      ioctl(tun_fd, TUNSETIFF, &req);
      ioctl(sock, SIOCSIFFLAGS, "gre0");
      write(tun_fd, "hi", 0);
      return 0;
    }
==========================

Signed-off-by: Alexander Potapenko <glider@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>