lightnvm: use system block for mm initialization

lightnvm: use system block for mm initialization

BugLink: http://bugs.launchpad.net/bugs/1531539
Use system block information to register the appropriate media manager.
This enables the LightNVM subsystem to instantiate a media manager
selected by the user, instead of relying on automatic detection by each
media manager loaded in the kernel.

A device must now be initialized before it can proceed to initialize its
media manager. Upon initialization, the configured media manager is
automatically initialized as well.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit b769207678176d590ea61ce7a64c9100925668b7)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>