core: fix data race in walk
core: fix data race in walk

This was detected by the race detector when the client of Walk() sorted entries.

yonjah/rclone,seuffert/rclone,ncw/rclone,ncw/rclone,yonjah/rclone,olihey/rclone,seuffert/rclone,seuffert/rclone,B4dM4n/rclone,B4dM4n/rclone,olihey/rclone,B4dM4n/rclone,B4dM4n/rclone,yonjah/rclone,seuffert/rclone,olihey/rclone,olihey/rclone,ncw/rclone,yonjah/rclone,ncw/rclone