arm64: mm: fix VA-range sanity check

In the Linux kernel, the following vulnerability has been resolved:

arm64: mm: fix VA-range sanity check

Both create_mapping_noalloc() and update_mapping_prot() sanity-check
their 'virt' parameter, but the check itself doesn't make much sense.
The condition used today appears to be a historical accident.

The sanity-check condition:

if ((virt >= PAGE_END) && (virt < VMALLOC_START)) {
[ ... warning here ... ]
return;
}

... can only be true for the KASAN shadow region or the module region,
and there's no reason to exclude these specifically for creating and
updateing mappings.

When arm64 support was first upstreamed in commit:

c1cc1552616d0f35 ("arm64: MMU initialisation")

... the condition was:

if (virt < VMALLOC_START) {
[ ... warning here ... ]
return;
}

At the time, VMALLOC_START was the lowest kernel address, and this was
checking whether 'virt' would be translated via TTBR1.

Subsequently in commit:

14c127c957c1c607 ("arm64: mm: Flip kernel VA space")

... the condition was changed to:

if ((virt >= VA_START) && (virt < VMALLOC_START)) {
[ ... warning here ... ]
return;
}

This appear to have been a thinko. The commit moved the linear map to
the bottom of the kernel address space, with VMALLOC_START being at the
halfway point. The old condition would warn for changes to the linear
map below this, and at the time VA_START was the end of the linear map.

Subsequently we cleaned up the naming of VA_START in commit:

77ad4ce69321abbe ("arm64: memory: rename VA_START to PAGE_END")

... keeping the erroneous condition as:

if ((virt >= PAGE_END) && (virt < VMALLOC_START)) {
[ ... warning here ... ]
return;
}

Correct the condition to check against the start of the TTBR1 address
space, which is currently PAGE_OFFSET. This simplifies the logic, and
more clearly matches the "outside kernel range" message in the warning.