file-posix: Drop s->lock_fd The lock_fd field is not strictly necessary because transferring locked bytes from old fd to the new one shouldn't fail anyway. This spares the user one fd per image. Signed-off-by: Fam Zheng <famz@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>