cpu_physical_memory_sync_dirty_bitmap: Fix alignment check

This code has an optimised, word aligned version, and a boring
unaligned version.  Recently 084140bd fixed a missing offset
addition from the core of both versions.  However, the offset isn't
necessarily aligned and thus the choice between the two versions
needs fixing up to also include the offset.

Symptom:
  A few stuck unsent pages during migration; not normally noticed
unless under very low bandwidth in which case the migration may get
stuck never ending and never performing a 2nd sync; noticed by
a hanging postcopy-test on a very heavily loaded system.

Fixes: 084140bd

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reported-by: Alex Benneé <alex.benee@linaro.org>
Tested-by: Alex Benneé <alex.benee@linaro.org>

--
v2
  Move 'page' inside the if (Comment from Paolo)
Message-Id: <20170724165125.29887-1-dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>