build: apply gofmt from golang 1.17

cmd/mount/test/seek_speed.go

@@ -1,3 +1,4 @@
+//go:build ignore
 // +build ignore
 
 // Read blocks out of a single file to time the seeking code

cmd/mount/test/seeker.go

@@ -1,3 +1,4 @@
+//go:build ignore
 // +build ignore
 
 // Read two files with lots of seeking to stress test the seek code

cmd/mount/test/seekers.go

@@ -1,3 +1,4 @@
+//go:build ignore
 // +build ignore
 
 // Read lots files with lots of simultaneous seeking to stress test the seek code