qcow2: Fix overly broad madvise()

@mem_size and @offset are both size_t, thus subtracting them from one
another will just return a big size_t if mem_size < offset -- even more
obvious here because the result is stored in another size_t.

Checking that result to be positive is therefore not sufficient to
exclude the case that offset > mem_size.  Thus, we currently sometimes
issue an madvise() over a very large address range.

This is triggered by iotest 163, but with -m64, this does not result in
tangible problems.  But with -m32, this test produces three segfaults,
all of which are fixed by this patch.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171114184127.24238-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
