Adds mapping_gfp_mask(), mapping_gfp_constraint(), mapping_set_unevictable(), folio_page() and folio_file_page(), which Linux's drm_gem_get_pages() uses. drm-kmod carries them in an overlay for virtio-gpu (freebsd/drm-kmod#517) today.

The mask is GFP_KERNEL, as shmem page allocation may sleep and ignores the other flags. folio_page() and folio_file_page() are exact copies from D58763, to not clash with it.

Builds on main for aarch64 and amd64: linuxkpi, linuxkpi_video, linuxkpi_wlan, and drm-kmod (drm, ttm, i915, amdgpu, radeon, virtio-gpu) without its overlay.

On 16.0-CURRENT aarch64 (main of Oct 1, UTM with virgl, WITNESS and INVARIANTS), virtio-gpu with a drm.ko built that way passes KMS, shmem buffer and EXECBUFFER stress, and sway with glmark2.