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/mount/fs.go

@@ -9,6 +9,7 @@ import (
 
 	"bazil.org/fuse"
 	fusefs "bazil.org/fuse/fs"
+	"github.com/ncw/rclone/cmd/mountlib"
 	"github.com/ncw/rclone/fs"
 	"github.com/ncw/rclone/fs/log"
 	"github.com/ncw/rclone/vfs"
@@ -72,6 +73,9 @@ func (f *FS) Statfs(ctx context.Context, req *fuse.StatfsRequest, resp *fuse.Sta
 	if free >= 0 {
 		resp.Bavail = uint64(free) / blockSize
 	}
+	mountlib.ClipBlocks(&resp.Blocks)
+	mountlib.ClipBlocks(&resp.Bfree)
+	mountlib.ClipBlocks(&resp.Bavail)
 	return nil
 }