linux-user: wrap fork() in a start/end exclusive section

When we do a fork() in usermode emulation, we need to be in
a start/end exclusive section, so that we can ensure that no
other thread is in an RCU section. Otherwise you can get this
deadlock:

- fork thread: has mmap_lock, waits for rcu_sync_lock
  (because rcu_init_lock() is registered as a pthread_atfork() hook)
- RCU thread: has rcu_sync_lock, waits for rcu_read_(un)lock
- another CPU thread: in RCU critical section, waits for mmap_lock

This can show up if you have a heavily multithreaded guest program
that does a fork().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Stuart Monteith <stuart.monteith@linaro.org>
Message-Id: <1512650481-1723-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
