dm kcopyd: update dm io interface

This patch ports kcopyd.c to the new, scalable dm_io() interface.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Heinz Mauelshagen <hjm@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>