Make upgrades to unsupported pre-feature flag pools trigger assertion

An assertion in spa.c is designed to prevent attempts to upgrade to
invalid pool versions.  It was written under the assumption that all
valid pools are less than SPA_VERSION. Since feature flags jumped from
28 to 5000, there any numbers in (28,5000) will fail to trigger the
assertion. We switch to the new SPA_VERSION_IS_SUPPORTED macro to
correct this.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>