mtd: Allocate bdi objects dynamically

mtd: Allocate bdi objects dynamically

The MTD backing dev info objects mtd_bdi was statically allocated.
So when MTD is built as a loadable module, this object fall in the
vmalloc address space.

The problem with that, is that the BDI APIs use wake_up_bit(), which calls
virt_to_page() to retrieve the memory zone of the page containing the
wait_queue to wake up, and virt_to_page() is not valid for vmalloc or
highmem addresses.

Fix this by allocating the BDI objects dynamically with kmalloc. The
objects now fall in the logical address space so that BDI APIs will
work in all cases (mtd builtin or module).

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Jim Baxter <jim_baxter@mentor.com>
Signed-off-by: Sandeep Jain <Sandeep_Jain@mentor.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>