spa.h: use IN_BASE instead of IN_FREEBSD_BASE

Consistently get the proper default value for autotrim.

Currently, only the kernel module is built with IN_FREEBSD_BASE,
and libzfs get the wrong default value, leading to confusion and
incorrect output when autotrim value was not set explicitly.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Yuri Pankov <yuripv@FreeBSD.org>
Closes #15016