KVM: selftests: Stash the host page size in a global in the guest_memfd test

KVM: selftests: Stash the host page size in a global in the guest_memfd test

Use a global variable to track the host page size in the guest_memfd test
so that the information doesn't need to be constantly passed around.  The
state is purely a reflection of the underlying system, i.e. can't be set
by the test and is constant for a given invocation of the test, and thus
explicitly passing the host page size to individual testcases adds no
value, e.g. doesn't allow testing different combinations.

Making page_size a global will simplify an upcoming change to create a new
guest_memfd instance per testcase.

No functional change intended.

Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>