target/s390x: Fix MVCRL with a large value in R0

target/s390x: Fix MVCRL with a large value in R0

Using a large R0 causes an assertion error:

    qemu-s390x: target/s390x/tcg/mem_helper.c:183: access_prepare_nf: Assertion `size > 0 && size <= 4096' failed.

Even though PoP explicitly advises against using more than 8 bits for the
size, an emulator crash is never a good thing.

Fix by truncating the size to 8 bits.

Fixes: ea0a1053e276 ("s390x/tcg: Implement Miscellaneous-Instruction-Extensions Facility 3 for the s390x")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: qemu-stable@nongnu.org
Message-Id: <20230704081506.276055-5-iii@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit 92a57534619a4058544ce8f9c0beae3e054f342b)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>