iscsi: Support unmapped I/O requests in the default initiator.

• Add icl_pdu_append_bio and icl_pdu_get_bio methods.

• When ICL_NOCOPY is used to append data from an unmapped I/O request to a PDU, construct unmapped mbufs from the relevant pages backing the struct bio.

• Use m_apply with a helper to compute crc32 digests on mbuf chains to handle unmapped mbufs. Since m_apply requires PMAP_HAS_DMAP for unmapped mbufs, only support unmapped requests when PMAP_HAS_DMAP is true.

Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D34406

(cherry picked from commit 530e725d8e9ba118ace2f75f010657a4fe4222a6)