Register notFound handler only once per root router (#6926)
Register notFound handler only once per root router (#6926)

registering notFound handler more than once causes
gorilla mux to return error for all registered paths
greater than > 8. This might be a bug in the gorilla/mux
but we shouldn't be using it this way. NotFound handler
should be only registered once per root router.

Fixes #6915
harshavardhana/minio,vadmeste/minio,NitishT/minio,harshavardhana/minio,jimjag/minio,jimjag/minio,vadmeste/minio,aead/minio,balamurugana/minio,krisis/minio,aead/minio,fwessels/minio,aead/minio,harshavardhana/minio,harshavardhana/minio,aead/minio,aead/minio,krishnasrinivas/minio,minio/minio,krishnasrinivas/minio,jimjag/minio,aead/minio,harshavardhana/minio,harshavardhana/minio,fwessels/minio,krishnasrinivas/minio,aead/minio,krisis/minio,harshavardhana/minio,donatello/minio,donatello/minio,krishnasrinivas/minio,NitishT/minio,aead/minio,minio/minio,NitishT/minio,harshavardhana/minio,fwessels/minio,NitishT/minio,balamurugana/minio,fwessels/minio,jimjag/minio