Fix various places which cast a pointer to a vm_paddr_t or vice versa.

GCC warns about the mismatched sizes on i386 where vm_paddr_t is 64
bits.

Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D36750

(cherry picked from commit a9fca3b987bcfa16d457b15cba2de46000f6bb86)