LinuxKPI: add memset_startat macro

Add a memset_startat() macro which sets a pattern from a struct member
to the end of the struct. Needed by a wireless driver.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37389

(cherry picked from commit 5d310ea8c5f98bbf53078ec632f6441f54dc9e44)