linuxkpi: Add freeram and freehigh to struct sysinfo

The struct layout is modified with this commit because new fields are
added in the middle, keeping original Linux order.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D37932

(cherry picked from commit e400b695991bf3f336e9cbe39e400943d8b1d676)