firewire: core: allocate the low memory region

Prevent userspace applications from allocating low memory address
ranges.  Otherwise, if some application happens to allocate such
a range and intends for a remote node to access it, and if that node
also implements SBP-2 (which will become more likely with the upcoming
SBP-2 target support), these accesses would be routed by the physical
DMA unit to some wrong memory address.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>