FreeBSD: Fix up after spa_stats.c move

Moving spa_stats added the additional burden of supporting
KSTAT_TYPE_IO.

spa_state_addr will always return a valid value regardless of
the value of 'n'. On FreeBSD this will cause an infinite loop
as it relies on the raw ops addr routine to indicate that there
is no more data.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10860