mount: remove nonseekable flag from write files - fixes #3461

Before this change rclone marked files opened for write without VFS
cache with the non seekable flag.

This caused problems with rclone mount layerd with mergerfs.

This change removes the hint and lets rclone do all the checking for
seekability.

cmd/mount/file.go

@@ -4,7 +4,6 @@ package mount
 
 import (
 	"context"
-	"io"
 	"time"
 
 	"bazil.org/fuse"
@@ -74,11 +73,6 @@ func (f *File) Open(ctx context.Context, req *fuse.OpenRequest, resp *fuse.OpenR
 		return nil, translateError(err)
 	}
 
-	// See if seeking is supported and set FUSE hint accordingly
-	if _, err = handle.Seek(0, io.SeekCurrent); err != nil {
-		resp.Flags |= fuse.OpenNonSeekable
-	}
-
 	return &FileHandle{handle}, nil
 }