Remove BIO_ORDERED flag from BIO_FLUSH sent by ZFS.

In all cases where ZFS sends BIO_FLUSH, it first waits for all related
writes to complete, so its BIO_FLUSH does not care about strict ordering.
Removal of one makes life much easier at least for NVMe driver, which
hardware has no concept of request ordering, relying completely on software.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.