MIPS: Fix modpost error in modules attepting to use virt_addr_valid().

ERROR: "min_low_pfn" [drivers/block/aoe/aoe.ko] undefined!

Fixed by moving the implementation of virt_addr_valid() into the kernel
proper and exporting it which removes the pains of an inline or macro
implementation.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>