arm: xen: foreign mapping PTEs are special.

These mappings are in fact special and require special handling in privcmd,
which already exists. Failure to mark the PTE as special on arm64 causes all
sorts of bad PTE fun. e.g.

e.g.:

BUG: Bad page map in process xl  pte:e0004077b33f53 pmd:4079575003
page:ffffffbce1a2f328 count:1 mapcount:-1 mapping:          (null) index:0x0
page flags: 0x4000000000000014(referenced|dirty)
addr:0000007fb5259000 vm_flags:040644fa anon_vma:          (null) mapping:ffffffc03a6fda58 index:0
vma->vm_ops->fault: privcmd_fault+0x0/0x38
vma->vm_file->f_op->mmap: privcmd_mmap+0x0/0x2c
CPU: 0 PID: 2657 Comm: xl Not tainted 3.12.0+ #102
Call trace:
[<ffffffc0000880f8>] dump_backtrace+0x0/0x12c
[<ffffffc000088238>] show_stack+0x14/0x1c
[<ffffffc0004b67e0>] dump_stack+0x70/0x90
[<ffffffc000125690>] print_bad_pte+0x12c/0x1bc
[<ffffffc0001268f4>] unmap_single_vma+0x4cc/0x700
[<ffffffc0001273b4>] unmap_vmas+0x68/0xb4
[<ffffffc00012c050>] unmap_region+0xcc/0x1d4
[<ffffffc00012df20>] do_munmap+0x218/0x314
[<ffffffc00012e060>] vm_munmap+0x44/0x64
[<ffffffc00012ed78>] SyS_munmap+0x24/0x34

Where unmap_single_vma contains inlined -> unmap_page_range -> zap_pud_range
-> zap_pmd_range -> zap_pte_range -> print_bad_pte.

Or:

BUG: Bad page state in process xl  pfn:4077b4d
page:ffffffbce1a2f8d8 count:0 mapcount:-1 mapping:          (null) index:0x0
page flags: 0x4000000000000014(referenced|dirty)
Modules linked in:
CPU: 0 PID: 2657 Comm: xl Tainted: G    B        3.12.0+ #102
Call trace:
[<ffffffc0000880f8>] dump_backtrace+0x0/0x12c
[<ffffffc000088238>] show_stack+0x14/0x1c
[<ffffffc0004b67e0>] dump_stack+0x70/0x90
[<ffffffc00010f798>] bad_page+0xc4/0x110
[<ffffffc00010f8b4>] free_pages_prepare+0xd0/0xd8
[<ffffffc000110e94>] free_hot_cold_page+0x28/0x178
[<ffffffc000111460>] free_hot_cold_page_list+0x38/0x60
[<ffffffc000114cf0>] release_pages+0x190/0x1dc
[<ffffffc00012c0e0>] unmap_region+0x15c/0x1d4
[<ffffffc00012df20>] do_munmap+0x218/0x314
[<ffffffc00012e060>] vm_munmap+0x44/0x64
[<ffffffc00012ed78>] SyS_munmap+0x24/0x34

x86 already gets this correct. 32-bit arm gets away with this because there is
not PTE_SPECIAL bit in the PTE there and the vm_normal_page fallback path does
the right thing.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>