vsock: Fix memory leak in vsock_connect()

An O_NONBLOCK vsock_connect() request may try to reschedule
@connect_work.  Imagine the following sequence of vsock_connect()
requests:

  1. The 1st, non-blocking request schedules @connect_work, which will
     expire after 200 jiffies.  Socket state is now SS_CONNECTING;

  2. Later, the 2nd, blocking request gets interrupted by a signal after
     a few jiffies while waiting for the connection to be established.
     Socket state is back to SS_UNCONNECTED, but @connect_work is still
     pending, and will expire after 100 jiffies.

  3. Now, the 3rd, non-blocking request tries to schedule @connect_work
     again.  Since @connect_work is already scheduled,
     schedule_delayed_work() silently returns.  sock_hold() is called
     twice, but sock_put() will only be called once in
     vsock_connect_timeout(), causing a memory leak reported by syzbot:

  BUG: memory leak
  unreferenced object 0xffff88810ea56a40 (size 1232):
    comm "syz-executor756", pid 3604, jiffies 4294947681 (age 12.350s)
    hex dump (first 32 bytes):
      00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
      28 00 07 40 00 00 00 00 00 00 00 00 00 00 00 00  (..@............
    backtrace:
      [<ffffffff837c830e>] sk_prot_alloc+0x3e/0x1b0 net/core/sock.c:1930
      [<ffffffff837cbe22>] sk_alloc+0x32/0x2e0 net/core/sock.c:1989
      [<ffffffff842ccf68>] __vsock_create.constprop.0+0x38/0x320 net/vmw_vsock/af_vsock.c:734
      [<ffffffff842ce8f1>] vsock_create+0xc1/0x2d0 net/vmw_vsock/af_vsock.c:2203
      [<ffffffff837c0cbb>] __sock_create+0x1ab/0x2b0 net/socket.c:1468
      [<ffffffff837c3acf>] sock_create net/socket.c:1519 [inline]
      [<ffffffff837c3acf>] __sys_socket+0x6f/0x140 net/socket.c:1561
      [<ffffffff837c3bba>] __do_sys_socket net/socket.c:1570 [inline]
      [<ffffffff837c3bba>] __se_sys_socket net/socket.c:1568 [inline]
      [<ffffffff837c3bba>] __x64_sys_socket+0x1a/0x20 net/socket.c:1568
      [<ffffffff84512815>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
      [<ffffffff84512815>] do_syscall_64+0x35/0x80 arch/x86/entry/common.c:80
      [<ffffffff84600068>] entry_SYSCALL_64_after_hwframe+0x44/0xae
  <...>

Use mod_delayed_work() instead: if @connect_work is already scheduled,
reschedule it, and undo sock_hold() to keep the reference count
balanced.

Reported-and-tested-by: syzbot+b03f55bf128f9a38f064@syzkaller.appspotmail.com
Fixes: d021c344051a ("VSOCK: Introduce VM Sockets")
Co-developed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
