integrate scsi_dh better into the scsi core V2

Christoph Hellwig
2015-05-10T18:42:14Z

This series ties scsi_dh deeper into the scsi core, and fixes all kinds
of issues in it, most importantly the race between using and detaching
device handlers.

Changes since V1:
- updated comments / strings based on review feedback
- moved scsi_dh.c to drivers/scsi to fix the srcdir = objdir build
- changed the old patch 2 to have saner handling of mismatching device
handlers in dm.
- dropped patch 1: not having a hw handler is fine if we don't plan to
change it anyway
- dropped patch 3: not necessary anymore.