linuxkpi: Add gfp_mask field to struct shrink_control

bz@ asked if the KBI breakage is a concern here. My answer was that this
is the first time in the DRM drivers in Linux 5.13 (the version I'm
working on) that this structure is initialized (as a variable local to
the function in this case), so it shouldn't be a problem for the DRM
drivers.

However, I can't speak for other drivers maintained outside of the src
tree.

Reviewed by: emaste, manu
Approved by: emaste, manu
Differential Revision: https://reviews.freebsd.org/D37913