Report "available" space rather than "free" space in volume stats

These two numbers may differ, and having the wrong number may result in
a volume having no useable space, while the metrics suggest it does.

bd2fs.py

@@ -124,15 +124,15 @@ class Bd2FsNodeServicer(csi_pb2_grpc.NodeServicer):
         return csi_pb2.NodeGetVolumeStatsResponse(
             usage=[
                 csi_pb2.VolumeUsage(
-                    available=stats["fs_free"],
+                    available=stats["fs_avail"],
                     total=stats["fs_size"],
-                    used=stats["fs_size"] - stats["fs_free"],
+                    used=stats["fs_size"] - stats["fs_avail"],
                     unit=csi_pb2.VolumeUsage.Unit.BYTES,
                 ),
                 csi_pb2.VolumeUsage(
-                    available=stats["fs_files_free"],
+                    available=stats["fs_files_avail"],
                     total=stats["fs_files"],
-                    used=stats["fs_files"] - stats["fs_files_free"],
+                    used=stats["fs_files"] - stats["fs_files_avail"],
                     unit=csi_pb2.VolumeUsage.Unit.INODES,
                 ),
             ]

metrics.py

@@ -10,9 +10,9 @@ def path_stats(path):
     fs_stat = os.statvfs(path)
     return {
         "fs_size": fs_stat.f_frsize * fs_stat.f_blocks,
-        "fs_free": fs_stat.f_frsize * fs_stat.f_bfree,
+        "fs_avail": fs_stat.f_frsize * fs_stat.f_bavail,
         "fs_files": fs_stat.f_files,
-        "fs_files_free": fs_stat.f_ffree,
+        "fs_files_avail": fs_stat.f_favail,
     }