fuse: adjust linux specific logic

Signed-off-by: Lei Liu <lei01.liu@horizon.ai>

weed/command/mount_std.go

@@ -87,6 +87,7 @@ func RunMount(filer, filerMountRootPath, dir, collection, replication, dataCente
 		}
 	}
 
+	// Ensure target mount point availability
 	if isValid := checkMountPointAvailable(dir); !isValid {
 		glog.Fatalf("Expected mount to still be active, target mount point: %s, please check!", dir)
 		return false