Merge the second part of the unmapped I/O changes.  This enables the
infrastructure in the block layer and UFS filesystem as well as a few
drivers.  The list of MFC revisions is long, so I won't quote changelogs.

r248508,248510,248511,248512,248514,248515,248516,248517,248518,
248519,248520,248521,248550,248568,248789,248790,249032,250936

Submitted by:	kib
Approved by:	kib
Obtained from:	Netflix