riscv: kexec: Fixup synchronization problem between init_mm and active_mm

The machine_kexec() uses set_memory_x to modify the direct mapping
attributes from RW to RWX. The current implementation of set_memory_x
does not split hugepages in the linear mapping and then when a PGD
mapping is used, the whole PGD is marked as executable. But changing
the permissions at the PGD level must be propagated to all the page
tables. When kexec jumps into control_buffer, the instruction page
fault happens, and there is no minor_pagefault for it, then panic.

The bug is found on an MMU_sv39 machine, and the direct mapping used a
1GB PUD, the pgd entries. Here is the bug output:

 kexec_core: Starting new kernel
 Will call new kernel at 00300000 from hart id 0
 FDT image at 747c7000
 Bye...
 Unable to handle kernel paging request at virtual address ffffffda23b0d000
 Oops [#1]
 Modules linked in:
 CPU: 0 PID: 53 Comm: uinit Not tainted 6.4.0-rc6 #15
 Hardware name: Sophgo Mango (DT)
 epc : 0xffffffda23b0d000
  ra : machine_kexec+0xa6/0xb0
 epc : ffffffda23b0d000 ra : ffffffff80008272 sp : ffffffc80c173d10
  gp : ffffffff8150e1e0 tp : ffffffd9073d2c40 t0 : 0000000000000000
  t1 : 0000000000000042 t2 : 6567616d69205444 s0 : ffffffc80c173d50
  s1 : ffffffd9076c4800 a0 : ffffffd9076c4800 a1 : 0000000000300000
  a2 : 00000000747c7000 a3 : 0000000000000000 a4 : ffffffd800000000
  a5 : 0000000000000000 a6 : ffffffd903619c40 a7 : ffffffffffffffff
  s2 : ffffffda23b0d000 s3 : 0000000000300000 s4 : 00000000747c7000
  s5 : 0000000000000000 s6 : 0000000000000000 s7 : 0000000000000000
  s8 : 0000000000000000 s9 : 0000000000000000 s10: 0000000000000000
  s11: 0000003f940001a0 t3 : ffffffff815351af t4 : ffffffff815351af
  t5 : ffffffff815351b0 t6 : ffffffc80c173b50
 status: 0000000200000100 badaddr: ffffffda23b0d000 cause: 000000000000000c

Given the current flaw in the set_memory_x implementation, the simplest
solution is to fix machine_kexec() to remap control code page outside
the linear mapping. Because the control code buffer was moved from the
direct mapping area to the vmalloc location, we need an additional
va_va_offset to fix up va_pa_offset.

Fixes: 3335068f ("riscv: Use PUD/P4D/PGD pages for the linear mapping")
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reported-by: Xing XiaoGuang <xingxg2008@163.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Tested-by: Xing Xiaoguang <xingxg2008@163.com>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>