ram: Let colo_flush_ram_cache take the bitmap_mutex

This is not required, colo_flush_ram_cache does not run concurrently
with the multifd threads since the cache is only flushed after
everything has been received. But it makes me more comfortable.

This will be used in the next commits to add colo support to multifd.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <35cb23ba854151d38a31e3a5c8a1020e4283cb4a.1683572883.git.lukasstraub2@web.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>