Fix regression in zfs_ereport_start()

Fix regression in zfs_ereport_start()

On 32-bit platforms spa_state is 32 bits without cast, and thus
caused a NULL pointer dereference when treated as 64bit in
var arg.  Accidentally introduced by bcdb96a.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Signed-off-by: Don Brady <don.brady@intel.com>
Closes #5966
Closes #5965