fix 'mmap' related race

[pabeni]

mptcp_connect self-tests with additional '-m mmap' argument produce a splat - WARN_ON_ONCE in mptcp_reset_timer() as the mptcp timer is not initialized yet

[cpaasch]

For the record, syzkaller reproducer is

# {Threaded:false Collide:false Repeat:false RepeatTimes:0 Procs:1 Sandbox: Fault:false FaultCall:-1 FaultNth:0 Leak:false NetInjection:false NetDevices:false NetReset:false Cgroups:false BinfmtMisc:false
CloseFDs:false KCSAN:false DevlinkPCI:false UseTmpDir:false HandleSegv:false Repro:false Trace:false}
r0 = socket$inet_mptcp(0x2, 0x1, 0x106)
r1 = socket$inet_mptcp(0x2, 0x1, 0x106)
bind$inet(r1, &(0x7f00000013c0)={0x2, 0x4e20, @multicast2}, 0x10)
connect$inet(r1, &(0x7f0000000040)={0x2, 0x0, @loopback}, 0x10)
listen(r1, 0x3)
connect$inet(r0, &(0x7f0000000040)={0x2, 0x4e20, @loopback}, 0x4d)
sendmsg$inet(r0, &(0x7f0000000280)={0x0, 0x0, &(0x7f0000000000)=[{&(0x7f0000000080)="ff", 0x20000081}], 0x1}, 0x0)

C-repro

// autogenerated by syzkaller (https://github.com/google/syzkaller)

#define _GNU_SOURCE

#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

uint64_t r[2] = {0xffffffffffffffff, 0xffffffffffffffff};

int main(void)
{
  syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 3ul, 0x32ul, -1, 0ul);
  intptr_t res = 0;
  res = syscall(__NR_socket, 2ul, 1ul, 0x106);
  if (res != -1)
    r[0] = res;
  res = syscall(__NR_socket, 2ul, 1ul, 0x106);
  if (res != -1)
    r[1] = res;
  *(uint16_t*)0x200013c0 = 2;
  *(uint16_t*)0x200013c2 = htobe16(0x4e20);
  *(uint32_t*)0x200013c4 = htobe32(0xe0000002);
  syscall(__NR_bind, r[1], 0x200013c0ul, 0x10ul);
  *(uint16_t*)0x20000040 = 2;
  *(uint16_t*)0x20000042 = htobe16(0);
  *(uint32_t*)0x20000044 = htobe32(0x7f000001);
  syscall(__NR_connect, r[1], 0x20000040ul, 0x10ul);
  syscall(__NR_listen, r[1], 3);
  *(uint16_t*)0x20000040 = 2;
  *(uint16_t*)0x20000042 = htobe16(0x4e20);
  *(uint32_t*)0x20000044 = htobe32(0x7f000001);
  syscall(__NR_connect, r[0], 0x20000040ul, 0x4dul);
  *(uint64_t*)0x20000280 = 0;
  *(uint32_t*)0x20000288 = 0;
  *(uint64_t*)0x20000290 = 0x20000000;
  *(uint64_t*)0x20000000 = 0x20000080;
  memcpy((void*)0x20000080, "\xff", 1);
  *(uint64_t*)0x20000008 = 0x20000081;
  *(uint64_t*)0x20000298 = 1;
  *(uint64_t*)0x200002a0 = 0;
  *(uint64_t*)0x200002a8 = 0;
  *(uint32_t*)0x200002b0 = 0;
  syscall(__NR_sendmsg, r[0], 0x20000280ul, 0ul);
  return 0;
}