syscall: Implement a convinience function restart_syscall

Currently when we have a signal pending we have the functionality
to restart that the current system call.  There are other cases
such as nasty lock ordering issues where it makes sense to have
a simple fix that uses try lock and restarts the system call.
Buying time to figure out how to rework the locking strategy.
Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sched.h

@@ -2178,6 +2178,12 @@ static inline int test_tsk_need_resched(struct task_struct *tsk)
 	return unlikely(test_tsk_thread_flag(tsk,TIF_NEED_RESCHED));
 }
 
+static inline int restart_syscall(void)
+{
+	set_tsk_thread_flag(current, TIF_SIGPENDING);
+	return -ERESTARTNOINTR;
+}
+
 static inline int signal_pending(struct task_struct *p)
 {
 	return unlikely(test_tsk_thread_flag(p,TIF_SIGPENDING));