LinuxKPI: Add want_init_on_free to linux/mm.h

want_init_on_free returns if heap memory zeroing on free is enabled.
FreeBSD does not zeroes heap memory on free().

Sponsored by: Serenity Cyber Security
Reviewed by: emaste
MFC after: 1 week

(cherry picked from commit 56041ee817fff844e5b2de7eedc447e388ab988e)