arm64: report correct stack pointer in KSTK_ESP for compat tasks

The KSTK_ESP macro is used to determine the user stack pointer for a
given task. In particular, this is used to to report the '[stack]' VMA
in /proc/self/maps, which is used by Android to determine the stack
location for children of the main thread.

This patch fixes the macro to use user_stack_pointer instead of directly
returning sp. This means that we report w13 instead of sp, since the
former is used as the stack pointer when executing in AArch32 state.

Cc: <[e-mail]>
Reported-by: Serban Constantinescu <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/include/asm/processor.h

···
 	((struct pt_regs *)(THREAD_START_SP + task_stack_page(p)) - 1)
 
 #define KSTK_EIP(tsk)	((unsigned long)task_pt_regs(tsk)->pc)
-#define KSTK_ESP(tsk)	((unsigned long)task_pt_regs(tsk)->sp)
+#define KSTK_ESP(tsk)	user_stack_pointer(task_pt_regs(tsk))
 
 /*
  * Prefetching support