OK, found it! maybe your Ubuntu version is 14 so its diferent kind of setup. I have Ubuntu 16.04 LTS and could be solved by modifing the parameter in: /etc/systemd/system/mysqld.service, its a symlink to: /lib/systemd/system/mariadb.service
# Number of files limit. previously [mysqld_safe] open-file-limit
#LimitNOFILE=16364
LimitNOFILE=40000
So here I replace to 40000 and no more warnings stating mariadb/mysql. Havent modified any other file, just this one and solved!
Hope it helps 