ZTS: Fix create-o_ashift test case

The function that fills the uberblock ring buffer on every device label
has been reworked to avoid occasional failures caused by a race
condition that prevents 'zpool sync' from writing some uberblock
sequentially: this happens when the pool sync ioctl dispatch code calls
txg_wait_synced() while we're already waiting for a TXG to sync.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6924
Closes #6977

cmd/zdb/zdb.c

@@ -2716,10 +2716,6 @@ dump_label(const char *dev)
 		exit(1);
 	}
 
-	if (ioctl(fd, BLKFLSBUF) != 0)
-		(void) printf("failed to invalidate cache '%s' : %s\n", path,
-		    strerror(errno));
-
 	if (fstat64_blk(fd, &statbuf) != 0) {
 		(void) printf("failed to stat '%s': %s\n", path,
 		    strerror(errno));
@@ -2727,6 +2723,10 @@ dump_label(const char *dev)
 		exit(1);
 	}
 
+	if (S_ISBLK(statbuf.st_mode) && ioctl(fd, BLKFLSBUF) != 0)
+		(void) printf("failed to invalidate cache '%s' : %s\n", path,
+		    strerror(errno));
+
 	avl_create(&config_tree, cksum_record_compare,
 	    sizeof (cksum_record_t), offsetof(cksum_record_t, link));
 	avl_create(&uberblock_tree, cksum_record_compare,