loader: Fix shadow_fb allocation
Using AllocateMaxAddress here means that gfx_state->tg_shadow_fb is
treated as the highest address we can receive. Since
gfx_state->tg_shadow_fb is NULL, we never receive anything. Use
AllocateAnyPages instead.
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1098