Add pipe_lock/unlock for splice_write to avoid oops by following competition:

(1) An application gets fds of a trace buffer, virtio-serial, pipe.
(2) The application does fork()
(3) The processes execute splice_read(trace buffer) and
    splice_write(virtio-serial) via same pipe.

        <parent>                   <child>
  get fds of a trace buffer,
         virtio-serial, pipe
          |
        fork()----------create--------+
          |                           |
      splice(read)                    |           ---+
      splice(write)                   |              +-- no competition
          |                       splice(read)       |
          |                       splice(write)   ---+
          |                           |
      splice(read)                    |
      splice(write)               splice(read)    ------ competition
          |                       splice(write)

Two processes share a pipe_inode_info structure. If the child execute
splice(read) when the parent tries to execute splice(write), the
structure can be broken. Existing virtio-serial driver does not get
lock for the structure in splice_write, so this competition will induce
oops.

<oops messages>
 BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
 IP: [<ffffffff811a6b5f>] splice_from_pipe_feed+0x6f/0x130
 PGD 7223e067 PUD 72391067 PMD 0
 Oops: 0000 [#1] SMP
 Modules linked in: lockd bnep bluetooth rfkill sunrpc ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 xt_state nf_conntrack ip6table_filter ip6_tables snd_hda_intel snd_hda_codec snd_hwdep snd_pcm snd_page_alloc snd_timer snd soundcore pcspkr virtio_net virtio_balloon i2c_piix4 i2c_core microcode uinput floppy
 CPU: 0 PID: 1072 Comm: compete-test Not tainted 3.10.0ws+ #55
 Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
 task: ffff880071b98000 ti: ffff88007b55e000 task.ti: ffff88007b55e000
 RIP: 0010:[<ffffffff811a6b5f>]  [<ffffffff811a6b5f>] splice_from_pipe_feed+0x6f/0x130
 RSP: 0018:ffff88007b55fd78  EFLAGS: 00010287
 RAX: 0000000000000000 RBX: ffff88007b55fe20 RCX: 0000000000000000
 RDX: 0000000000001000 RSI: ffff88007a95ba30 RDI: ffff880036f9e6c0
 RBP: ffff88007b55fda8 R08: 00000000000006ec R09: ffff880077626708
 R10: 0000000000000003 R11: ffffffff8139ca59 R12: ffff88007a95ba30
 R13: 0000000000000000 R14: ffffffff8139dd00 R15: ffff880036f9e6c0
 FS:  00007f2e2e3a0740(0000) GS:ffff88007fc00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
 CR2: 0000000000000018 CR3: 0000000071bd1000 CR4: 00000000000006f0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
 Stack:
  ffffffff8139ca59 ffff88007b55fe20 ffff880036f9e6c0 ffffffff8139dd00
  ffff8800776266c0 ffff880077626708 ffff88007b55fde8 ffffffff811a6e8e
  ffff88007b55fde8 ffffffff8139ca59 ffff880036f9e6c0 ffff88007b55fe20
 Call Trace:
  [<ffffffff8139ca59>] ? alloc_buf.isra.13+0x39/0xb0
  [<ffffffff8139dd00>] ? virtcons_restore+0x100/0x100
  [<ffffffff811a6e8e>] __splice_from_pipe+0x7e/0x90
  [<ffffffff8139ca59>] ? alloc_buf.isra.13+0x39/0xb0
  [<ffffffff8139d739>] port_fops_splice_write+0xe9/0x140
  [<ffffffff8127a3f4>] ? selinux_file_permission+0xc4/0x120
  [<ffffffff8139d650>] ? wait_port_writable+0x1b0/0x1b0
  [<ffffffff811a6fe0>] do_splice_from+0xa0/0x110
  [<ffffffff811a951f>] SyS_splice+0x5ff/0x6b0
  [<ffffffff8161facf>] tracesys+0xdd/0xe2
 Code: 49 8b 87 80 00 00 00 4c 8d 24 d0 8b 53 04 41 8b 44 24 0c 4d 8b 6c 24 10 39 d0 89 03 76 02 89 13 49 8b 44 24 10 4c 89 e6 4c 89 ff <ff> 50 18 85 c0 0f 85 aa 00 00 00 48 89 da 4c 89 e6 4c 89 ff 41
 RIP  [<ffffffff811a6b5f>] splice_from_pipe_feed+0x6f/0x130
  RSP <ffff88007b55fd78>
 CR2: 0000000000000018
 ---[ end trace 24572beb7764de59 ]---

V2: Fix a locking problem for error
V3: Add Reviewed-by lines and stable@ line in sign-off area
Signed-off-by: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Amit Shah <amit.shah@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>