arm64: fix missing syscall trace exit

If a syscall is entered without TIF_SYSCALL_TRACE set, then it goes on
the fast path.  It's then possible to have TIF_SYSCALL_TRACE added in
the middle of the syscall, but ret_fast_syscall doesn't check this flag
again.  This causes a ptrace syscall-exit-stop to be missed.

For instance, from a PTRACE_EVENT_FORK reported during do_fork, the
tracer might resume with PTRACE_SYSCALL, setting TIF_SYSCALL_TRACE.
Now the completion of the fork should have a syscall-exit-stop.

Russell King fixed this on arm by re-checking _TIF_SYSCALL_WORK in the
fast exit path.  Do the same on arm64.
Reviewed-by: Will Deacon <will.deacon@arm.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Josh Stone <jistone@redhat.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/entry.S

@@ -611,11 +611,16 @@ ENDPROC(cpu_switch_to)
  */
 ret_fast_syscall:
 	disable_irq				// disable interrupts
-	ldr	x1, [tsk, #TI_FLAGS]
+	ldr	x1, [tsk, #TI_FLAGS]		// re-check for syscall tracing
+	and	x2, x1, #_TIF_SYSCALL_WORK
+	cbnz	x2, ret_fast_syscall_trace
 	and	x2, x1, #_TIF_WORK_MASK
 	cbnz	x2, fast_work_pending
 	enable_step_tsk x1, x2
 	kernel_exit 0, ret = 1
+ret_fast_syscall_trace:
+	enable_irq				// enable interrupts
+	b	__sys_trace_return
 
 /*
  * Ok, we need to do extra processing, enter the slow path.