ARM: fix missing syscall trace exit

Josh Stone reports:

I've discovered a case where both arm and arm64 will miss a ptrace
syscall-exit that they should report. If the 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.

Fix this by always checking for a syscall trace in the fast exit path.

Reported-by: Josh Stone <[e-mail]>
Signed-off-by: Russell King <[e-mail]>

arch/arm/kernel/entry-common.S

···
  UNWIND(.fnstart	)
  UNWIND(.cantunwind	)
 	disable_irq				@ disable interrupts
-	ldr	r1, [tsk, #TI_FLAGS]
+	ldr	r1, [tsk, #TI_FLAGS]		@ re-check for syscall tracing
+	tst	r1, #_TIF_SYSCALL_WORK
+	bne	__sys_trace_return
 	tst	r1, #_TIF_WORK_MASK
 	bne	fast_work_pending
 	asm_trace_hardirqs_on