new helper: current_pt_regs()

Normally (and that's the default) it's just task_pt_regs(current).
However, if an architecture can optimize that, it can do so by
making a macro of its own available from asm/ptrace.h.  More
importantly, some architectures have task_pt_regs() working only
for traced tasks blocked on signal delivery.  current_pt_regs()
needs to work for *all* processes, so before those architectures
start using stuff relying on current_pt_regs() they'll need a
properly working variant.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/ptrace.h

@@ -403,6 +403,10 @@ static inline void user_single_step_siginfo(struct task_struct *tsk,
 #define arch_ptrace_stop(code, info)		do { } while (0)
 #endif
 
+#ifndef current_pt_regs
+#define current_pt_regs() task_pt_regs(current)
+#endif
+
 extern int task_current_syscall(struct task_struct *target, long *callno,
 				unsigned long args[6], unsigned int maxargs,
 				unsigned long *sp, unsigned long *pc);