btrfs-progs: judge the return value of check_mounted more accurately

For btrfs-convert, btrfstune, btrfs rescue, they report "device busy"
when given a device that does not actually exist e.g.

	# btrfstune -x abcdefg (this device does not exist)
	$ ...device busy...

We deal with this case by add "ret < 0" error check when
judging the return value of check_mounted.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

btrfs-convert.c

@@ -2729,7 +2729,12 @@ int main(int argc, char *argv[])
 	}
 
 	file = argv[optind];
-	if (check_mounted(file)) {
+	ret = check_mounted(file);
+	if (ret < 0) {
+		fprintf(stderr, "Could not check mount status: %s\n",
+			strerror(-ret));
+		return 1;
+	} else if (ret) {
 		fprintf(stderr, "%s is mounted\n", file);
 		return 1;
 	}

btrfstune.c

@@ -151,7 +151,12 @@ int main(int argc, char *argv[])
 		return 1;
 	}
 
-	if (check_mounted(device)) {
+	ret = check_mounted(device);
+	if (ret < 0) {
+		fprintf(stderr, "Could not check mount status: %s\n",
+			strerror(-ret));
+		return 1;
+	} else if (ret) {
 		fprintf(stderr, "%s is mounted\n", device);
 		return 1;
 	}

cmds-rescue.c

@@ -80,9 +80,13 @@ int cmd_chunk_recover(int argc, char *argv[])
 	file = argv[optind];
 
 	ret = check_mounted(file);
-	if (ret) {
+	if (ret < 0) {
+		fprintf(stderr, "Could not check mount status: %s\n",
+			strerror(-ret));
+		return 1;
+	} else if (ret) {
 		fprintf(stderr, "the device is busy\n");
-		return ret;
+		return 1;
 	}
 
 	ret = btrfs_recover_chunk_tree(file, verbose, yes);
@@ -133,7 +137,11 @@ int cmd_super_recover(int argc, char **argv)
 
 	dname = argv[optind];
 	ret = check_mounted(dname);
-	if (ret) {
+	if (ret < 0) {
+		fprintf(stderr, "Could not check mount status: %s\n",
+			strerror(-ret));
+		return 1;
+	} else if (ret) {
 		fprintf(stderr, "the device is busy\n");
 		return 1;
 	}