vfs: allow ReadAt and WriteAt to run concurrently with themselves
vfs: allow ReadAt and WriteAt to run concurrently with themselves

This should help with throughput on mounts and help when multiple
readers have the file open.

See: https://forum.rclone.org/t/concurrent-read-accesses-on-the-same-file-through-rclone-vfs-mount/17192

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone