stat64_min_slow() and stat64_max_slow() compare the wrong way.  This
makes iotest 136 fail with clang and -m32.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20171114232223.25207-1-mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>