Some architectures (powerpc Book-E) have a vm_paddr_t larger than intptr_t.
Casting from the intptr_t to vm_paddr_t causes sign extension, leading to a
potentially invalid address.

This was seen when running X on a PowerPC P1022 machine, which mapped the
backing framebuffer at 0xc1800000. When mmap()d by X, this yielded an invalid
address of 0xffffffffc1800000, or, as the hardware would see it, 0xfc1800000.