linuxkpi: Include <linux/vmalloc.h> from <linux/pagemap.h>

I'm not sure this is correct, but it gives access to the definition of
PAGE_KERNEL in ttm_tt.c in the DRM drivers.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38154

(cherry picked from commit 3d47e3bf7b3725482d6ae01675db097a6de020b6)