Virtual memory map to accomodate 64 MB SPI flash

We use a 64 MB SPI NOR flash on Prowl which the SPI flash driver
ioremaps() *twice* into the vmalloc area. As a result, we needed to
increase the size of the vmalloc area from 64MB to 256MB. The existing
virtual memory layout did not accommodate this change. We had to tweak
it a little. Note that TASK_UNMAPPED_BASE needs to be a multiple of
1 file changed