This patch rewrites consistent dma allocations support to use vmalloc
layer to allocate virtual memory space from vmalloc pool and get rid
of CONFIG_CONSISTENT_{START,SIZE}.

This greatly simplifies the code by effectively removing a custom
allocator we had for virtual space.
Signed-off-by: NIlya Yanok <yanok@emcraft.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>