x86/crash: don't use set_fixmap() in the crash path

x86/crash: don't use set_fixmap() in the crash path

Experimentally, this can result in memory allocation, and in particular a
failed assertion that interrupts are enabled when performing a TLB flush.

  (XEN) Assertion 'local_irq_is_enabled()' failed at smp.c:223
  <snip>
  (XEN) [<ffff82d08018a0d3>] flush_area_mask+0x7/0x134
  (XEN) [<ffff82d08011f7c6>] alloc_domheap_pages+0xa9/0x12a
  (XEN) [<ffff82d08011f8ab>] alloc_xenheap_pages+0x64/0xdb
  (XEN) [<ffff82d080178e08>] alloc_xen_pagetable+0x1c/0xa0
  (XEN) [<ffff82d08017926b>] virt_to_xen_l1e+0x38/0x1be
  (XEN) [<ffff82d080179bff>] map_pages_to_xen+0x80e/0xfd9
  (XEN) [<ffff82d080185a23>] __set_fixmap+0x2c/0x2e
  (XEN) [<ffff82d0801a6fd4>] machine_crash_shutdown+0x186/0x2b2
  (XEN) [<ffff82d0801172bb>] kexec_crash+0x3f/0x5b
  (XEN) [<ffff82d0801479b7>] panic+0x100/0x118
  (XEN) [<ffff82d08019002b>] set_guest_machinecheck_trapbounce+0/0x6d
  (XEN) [<ffff82d080195c15>] do_page_fault+0x40b/0x541
  (XEN) [<ffff82d0802345e0>] handle_exception_saved+0x2e/0x6c

Instead, use the directmap mapping which are writable and involve far less
complexity than set_fixmap()

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>