Initialize "autoreplace" in spa_ld_get_props()

spa_prop_find() may fail to find the specified property, in which case
it suppresses ENOENT from zap_lookup().  In this case, the return value
is left uninitialized, so spa_autoreplace was being initialized using an
uninitialized stack variable.

This was found using KMSAN.  It appears to be a regression from commit
9eb7b46, which removed the initialization of "autoreplace" from the
definition.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes openzfs#12383

module/zfs/spa.c

@@ -4183,7 +4183,7 @@ spa_ld_get_props(spa_t *spa)
 		return (spa_vdev_err(rvd, VDEV_AUX_CORRUPT_DATA, EIO));
 
 	if (error == 0) {
-		uint64_t autoreplace;
+		uint64_t autoreplace = 0;
 
 		spa_prop_find(spa, ZPOOL_PROP_BOOTFS, &spa->spa_bootfs);
 		spa_prop_find(spa, ZPOOL_PROP_AUTOREPLACE, &autoreplace);