Fix zap_lookup() in feature_is_supported().

The length (number of integers) argument passed to zap_lookup was wrong;
likely as a result of performing stack-reduction on the function.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2141