KVM: s390x: selftests: Add shared zeropage test

Let's test that we can have shared zeropages in our process as long as
storage keys are not getting used, that shared zeropages are properly
unshared (replaced by anonymous pages) once storage keys are enabled,
and that no new shared zeropages are populated after storage keys
were enabled.

We require the new pagemap interface to detect the shared zeropage.

On an old kernel (zeropages always disabled):
	# ./s390x/shared_zeropage_test
	TAP version 13
	1..3
	not ok 1 Shared zeropages should be enabled
	ok 2 Shared zeropage should be gone
	ok 3 Shared zeropages should be disabled
	# Totals: pass:2 fail:1 xfail:0 xpass:0 skip:0 error:0

On a fixed kernel:
	# ./s390x/shared_zeropage_test
	TAP version 13
	1..3
	ok 1 Shared zeropages should be enabled
	ok 2 Shared zeropage should be gone
	ok 3 Shared zeropages should be disabled
	# Totals: pass:3 fail:0 xfail:0 xpass:0 skip:0 error:0

Testing of UFFDIO_ZEROPAGE can be added later.

[ agordeev: Fixed checkpatch complaint, added ucall_common.h include ]

Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Acked-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Tested-by: Alexander Gordeev <agordeev@linux.ibm.com>
Link: https://lore.kernel.org/r/20240412084329.30315-1-david@redhat.com
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>