Fix sa_legacy_attr_count to use ARRAY_SIZE

Replace magic value 16 with ARRAY_SIZE() to correctly handle
when the sa_legacy_attrs array size changes.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: cao.xuewen <cao.xuewen@zte.com.cn>
Closes #5354