migration/multifd: Zero p->flags before starting filling a packet

migration/multifd: Zero p->flags before starting filling a packet

This way there aren't stale flags there.

p->flags can't contain SYNC to be sent at the next RAM packet since syncs
are now handled separately in multifd_send_thread.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Link: https://lore.kernel.org/r/1c96b6cdb797e6f035eb1a4ad9bfc24f4c7f5df8.1730203967.git.maciej.szmigiero@oracle.com
Signed-off-by: Peter Xu <peterx@redhat.com>