KVM: selftests: Add guest_memfd regression test signed offset+size bug

Add a regression (and proof-of-bug) testcase to ensure KVM rejects an
offset+size that would result in a negative value when computed as a signed
64-bit value.  KVM had a flaw where it would allow binding a memslot to a
guest_memfd instance even with a wildly out-of-range offset, if the offset
and size were both positive values, but the combined offset+size was
negative.

Use "0x7fffffffffffffffull - page_size", i.e. "INT64_MAX - page_size", for
the offset as the size of the guest_memfd file must be at least page_size
(KVM requires memslots and gmem files to be host page-size aligned).  I.e.
"INT64_MAX - page_size + size" is guaranteed to generate an offset+size
that is negative when converted to a signed 64-bit value *and* honors KVM's
alignment requirements.

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Link: https://patch.msgid.link/20260602170921.1304394-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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