[POWERPC] Fix ppc32 single-stepping out of syscalls

The ppc32 kernel didn't properly set/clear the TIF_SINGLESTEP
flag, causing return from syscalls to not SIGTRAP, thus executing
one more instruction before stopping again.

This fixes it.  The ptrace code is a bit of a mess, and is overdue
for at least a -proper- 32/64 bits split and possibly more cleanups
but this minimum fix should be ok for 2.6.22

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/ptrace.c

@@ -218,6 +218,7 @@ set_single_step(struct task_struct *task)
 		regs->msr |= MSR_SE;
 #endif
 	}
+	set_tsk_thread_flag(task, TIF_SINGLESTEP);
 }
 
 static inline void
@@ -233,6 +234,7 @@ clear_single_step(struct task_struct *task)
 		regs->msr &= ~MSR_SE;
 #endif
 	}
+	clear_tsk_thread_flag(task, TIF_SINGLESTEP);
 }
 #endif /* CONFIG_PPC32 */