Prevent gcc -Werror=maybe-uninitialized warnings in spa_wait_common()

This commit fixes the following build failure detected on Debian9
(GCC 6.3.0):

     CC [M]  module/zfs/spa.o
   module/zfs/spa.c: In function ‘spa_wait_common.part.31’:
   module/zfs/spa.c:9468:6: error: ‘in_progress’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
      if (!in_progress || spa->spa_waiters_cancel || error)
         ^
   cc1: all warnings being treated as errors

Reviewed-by: Chris Dunlop <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: John Gallagher <[email protected]>
Signed-off-by: loli10K <[email protected]>
Closes #9326

module/zfs/spa.c

@@ -9465,7 +9465,7 @@ spa_wait_common(const char *pool, zpool_wait_activity_t activity,
 		error = spa_activity_in_progress(spa, activity, use_tag, tag,
 		    &in_progress);
 
-		if (!in_progress || spa->spa_waiters_cancel || error)
+		if (error || !in_progress || spa->spa_waiters_cancel)
 			break;
 
 		*waited = B_TRUE;