LinuxKPI: add ALIGN_DOWN()

Add ALIGN_DOWN as rounddown2() along ALIGN() which is implemented as
roundup2().

Sponsored by: The FreeBSD Foundation
Reviewed by: manu, hselasky (now with less ())
Differential Revision: https://reviews.freebsd.org/D34844

(cherry picked from commit 97f2e93a66a804ae9b6187998dbc4427b647e274)