migration: Remove RAMState.f references in compression code

migration: Remove RAMState.f references in compression code

Removing referencing to RAMState.f in compress_page_with_multi_thread() and
flush_compressed_data().

Compression code by default isn't compatible with having >1 channels (or it
won't currently know which channel to flush the compressed data), so to
make it simple we always flush on the default to_dst_file port until
someone wants to add >1 ports support, as rs->f right now can really
change (after postcopy preempt is introduced).

There should be no functional change at all after patch applied, since as
long as rs->f referenced in compression code, it must be to_dst_file.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>