arm64: fix retrieving pt_regs structure on the kernel stack

Currently, bt command in gdb shows broken backtrace as follows:

    Core was generated by `/init'.
     #0  0x0000000000000015 in ?? ()

Then, registers contain broken values as follows:

    (gdb) info registers
    x0             0xffffff80080e1d3c -549620671172
    ...
    x26            0xc79ecaf 209317039
    x27            0xef412c356afbd300 -1206634617218804992
    x28            0x7 7
    x29            0xef412c356afbd300 -1206634617218804992
    x30            0x16 22
    sp             0x124 0x124
    pc             0x15 0x15
    cpsr           0x40000000 [ EL=0 Z ]
    fpsr           0x10 16
    fpcr           0x0 0

This is because the way retrieving pt_regs structure on the kernel
stack is wrong. The subtraction of SIZE(pt_regs) from the address is
unnecessary.

The cause of this issue is that I added a bug when I merged the commit
19bfb92; the Zhao's original patch
was totally correct.

Signed-off-by: Hong YANG <hong.yang3@nio.com>
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: HATAYAMA Daisuke <d.hatayama@fujitsu.com>

Author: d-hatayama

src/libgcore/gcore_arm64.c

@@ -29,7 +29,7 @@ static int gpr_get(struct task_context *target,
 	BZERO(regs, sizeof(*regs));
 
 	readmem(machdep->get_stacktop(target->task) -
-		machdep->machspec->user_eframe_offset - SIZE(pt_regs), KVADDR,
+		machdep->machspec->user_eframe_offset, KVADDR,
 		regs, sizeof(struct user_pt_regs), "gpr_get: user_pt_regs",
 		gcore_verbose_error_handle());
 
@@ -126,7 +126,7 @@ static int compat_gpr_get(struct task_context *target,
 	BZERO(regs, sizeof(*regs));
 
 	readmem(machdep->get_stacktop(target->task) -
-		machdep->machspec->user_eframe_offset - SIZE(pt_regs), KVADDR,
+		machdep->machspec->user_eframe_offset, KVADDR,
 		&pt_regs, sizeof(struct pt_regs), "compat_gpr_get: pt_regs",
 		gcore_verbose_error_handle());