mirror: limit niov to IOV_MAX elements, again

During the refactor of mirror_iteration in e5b43573,
we regressed the fix introduced in cae98cb8.

This patch re-adds IOV_MAX checking to cases where we
aren't checking alignment (and size) already.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1466625064-11280-3-git-send-email-jsnow@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
