gcc9: quiet Waddress-of-packed-member for user build

Disable the warning for WARNS <= 3. This is lame, but it's what we
already do for the clang build.

Reviewed by: dim
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22889

(cherry picked from commit fc41af14c8298c88d5a2bee85ecf1864a3d37a51)