parser: add unsigned int parser

Will be used by fs parsing options

Link: https://lore.kernel.org/r/20210129045242.10268-1-bingjingc@synology.com


Reviewed-by: Robbie <Ko&lt;robbieko@synology.com>
Reviewed-by: Chung-Chiang Cheng <cccheng@synology.com>
Signed-off-by: BingJing Chang <bingjingc@synology.com>
Signed-off-by: Jan Kara <jack@suse.cz>