smode_trap_vector函数是否忽略了寄存器t6

用户态转换到内核态时调用`smode_trap_vector`，需要保存当前状态下的所有寄存器等环境。
给出的代码是这样的：
```asm
smode_trap_vector:
    # swap a0 and sscratch, so that points a0 to the trapframe of current process
    csrrw a0, sscratch, a0

# save the context (user registers) of current process in its trapframe.
    addi t6, a0 , 0
    store_all_registers

# come back to save a0 register before entering trap handling in trapframe
    csrr t0, sscratch
    sd t0, 72(a0)

# use the "user kernel" stack (whose pointer stored in p->trapframe->kernel_sp)
    ld sp, 248(a0)

# load the address of smode_trap_handler() from p->trapframe->kernel_trap
    ld t0, 256(a0)

# jump to smode_trap_handler() that is defined in kernel/trap.c
    jr t0
```

`store_all_registers` 函数使用t6作为基址, 我的理解是最后没有保存t6之前的值；
这里实际上应该是可以不用a0寄存器的吧。

改成这样合理吗：
```asm
smode_trap_vector:
    # swap a0 and sscratch, so that points a0 to the trapframe of current process
    csrrw t6, sscratch, t6

# save the context (user registers) of current process in its trapframe.
    # addi t6, a0 , 0
    store_all_registers

# come back to save a0 register before entering trap handling in trapframe
    csrr t0, sscratch
    sd t0, 240(t6)

# use the "user kernel" stack (whose pointer stored in p->trapframe->kernel_sp)
    ld sp, 248(t6)

# load the address of smode_trap_handler() from p->trapframe->kernel_trap
    ld t0, 256(t6)

# jump to smode_trap_handler() that is defined in kernel/trap.c
    jr t0
```

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