target/s390x/arch_dump: Fix memory corruption in s390x_write_elf64_notes()

target/s390x/arch_dump: Fix memory corruption in s390x_write_elf64_notes()

"note_size" can be smaller than sizeof(note), so unconditionally calling
memset(notep, 0, sizeof(note)) could cause a memory corruption here in
case notep has been allocated dynamically, thus let's use note_size as
length argument for memset() instead.

Reported-by: Sebastian Mitterle <smitterl@redhat.com>
Fixes: 113d8f4e95 ("s390x: pv: Add dump support")
Message-Id: <20230214141056.680969-1-thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>