rcu: Fix unrecovered RCU user mode in syscall_trace_leave()

On x86-64 syscall exit, 3 non exclusive events may happen
looping in the following order:

1) Check if we need resched for user preemption, if so call
schedule_user()

2) Check if we have pending signals, if so call do_notify_resume()

3) Check if we do syscall tracing, if so call syscall_trace_leave()

However syscall_trace_leave() has been written assuming it directly
follows the syscall and forget about the above possible 1st and 2nd
steps.

Now schedule_user() and do_notify_resume() exit in RCU user mode
because they have most chances to resume userspace immediately and
this avoids an rcu_user_enter() call in the syscall fast path.

So by the time we call syscall_trace_leave(), we may well be in RCU
user mode. To fix this up, simply call rcu_user_exit() in the beginning
of this function.

This fixes some reported RCU uses in extended quiescent state.

Reported-by: Dave Jones <davej@redhat.com>
Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Tested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>