Revert "openzfs: use IN_BASE instead of IN_FREEBSD_BASE in spa.h"

Further testing (sadly, after committing) shows that I missed the fact
that IN_BASE is used as userland/kernel delimiter (and not just for
FreeBSD-specific code unlike the IN_FREEBSD_BASE). Revert until I have
a full (and proper) fix.

This reverts commit d2a45e9e817ad68f3e163d13404744b8ea9c675b.