mount,cmount: clip the number of blocks to 2^32-1 on macOS

OSX FUSE only supports 32 bit number of blocks which means that block
counts have been wrapping.  This causes f_bavail to be 0 which in turn
causes problems with programs like borg backup.

Fixes #2356

cmd/mountlib/mount.go

@@ -293,3 +293,22 @@ be copied to the vfs cache before opening with --vfs-cache-mode full.
 
 	return commandDefintion
 }
+
+// ClipBlocks clips the blocks pointed to to the OS max
+func ClipBlocks(b *uint64) {
+	var max uint64
+	switch runtime.GOOS {
+	case "windows":
+		max = (1 << 43) - 1
+	case "darwin":
+		// OSX FUSE only supports 32 bit number of blocks
+		// https://github.com/osxfuse/osxfuse/issues/396
+		max = (1 << 32) - 1
+	default:
+		// no clipping
+		return
+	}
+	if *b > max {
+		*b = max
+	}
+}