Add unconvert linter

This linter checks for unnecessary type convertions.

Some convertions are whitelisted because their type is different
on 32bit platforms

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

content/local/store_unix.go

@@ -12,8 +12,7 @@ import (
 
 func getATime(fi os.FileInfo) time.Time {
 	if st, ok := fi.Sys().(*syscall.Stat_t); ok {
-		return time.Unix(int64(sys.StatAtime(st).Sec),
-			int64(sys.StatAtime(st).Nsec))
+		return sys.StatATimeAsTime(st)
 	}
 
 	return fi.ModTime()