kernel: Fix bus_dmamem_alloc()

Its call to pmap_change_attr() contained a typo which caused kernel
panics on some machines.

Reported-by: Markus Pfeiffer
With-advice-from: Venkatesh Srinivas
kernel - Fix /dev/mem

* Fix offset vs page-index mistake for old-style device mmap.  This
  fixes /dev/mem and programs that use it like acpidump.

* There may be other issues.

Reported-by: David Shao bug #2576
kernel - Fix panic when usched is used to force a cpu w/the dfly scheduler

* Fix a panic for 'usched dfly:0x1 sh', or other similar cpu forcing
  mechanic.

* The scheduler was not being notified of the forced migration which
  caused it to assert on a sanity check later on.  Add the needed
  infrastructure.

Reported-by: vsrinivas
kernel/amr: Move amrd (disk) adding before amrp (cam passthrough).

This fixes a panic seen when amr/amrp were compiled into the kernel.

Dragonfly-bug: <http://bugs.dragonflybsd.org/issues/2575>
Reported-by:   justin
Actual-fix-by: sephe