new helper: current_user_stack_pointer()

Cross-architecture equivalent of rdusp(); default is
user_stack_pointer(current_pt_regs()) - that works for almost all
platforms that have usp saved in pt_regs.  The only exception from
that is ia64 - we want memory stack, not the backing store for
register one.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/blackfin/include/asm/ptrace.h

@@ -106,6 +106,7 @@ struct pt_regs {
 #define arch_has_single_step()	(1)
 /* common code demands this function */
 #define ptrace_disable(child) user_disable_single_step(child)
+#define current_user_stack_pointer() rdusp()
 
 extern int is_user_addr_valid(struct task_struct *child,
 			      unsigned long start, unsigned long len);