target/s390x: Fix VSTL with a large length

target/s390x: Fix VSTL with a large length

The length is always truncated to 16 bytes. Do not probe more than
that.

Cc: qemu-stable@nongnu.org
Fixes: 0e0a5b49ad58 ("s390x/tcg: Implement VECTOR STORE WITH LENGTH")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230804235624.263260-1-iii@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit 6db3518ba4fcddd71049718f138552999f0d97b4)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>