exec: check offset_within_address_space for register subpage

If offset_within_address_space falls in a page, then we register a
subpage. So check offset_within_address_space rather than
offset_within_region.

Cc: qemu-stable@nongnu.org
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: "Andreas Färber" <afaerber@suse.de>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: NHu Tao <hutao@cn.fujitsu.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -854,7 +854,7 @@ static void mem_add(MemoryListener *listener, MemoryRegionSection *section)
         now = remain;
         if (int128_lt(remain.size, page_size)) {
             register_subpage(d, &now);
-        } else if (remain.offset_within_region & ~TARGET_PAGE_MASK) {
+        } else if (remain.offset_within_address_space & ~TARGET_PAGE_MASK) {
             now.size = page_size;
             register_subpage(d, &now);
         } else {