openzfs: use IN_BASE instead of IN_FREEBSD_BASE in spa.h

Consistently use IN_BASE to allow libzfs to get the same default
autotrim value as kernel does.

Note that this does not change the default value itself, rather
fixing the source of value and the value itself in e.g. zpool get
output if it was not set explicitly. (And as a reminder, default
value of autotrim on FreeBSD is 'on', despite what zpoolprops(7)
says currently.)

PR: 264234
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40075