`uninit_bg` and lazy block group allocation in EXT3/4

_E_xt3 and Ext4 recently have been the most commonly used file system on Linux machines.

What does uninit_bg actually do?

Read https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout

section `Meta Block Groups and Lazy Block Group Initialization`.

https://ext4.wiki.kernel.org/index.php/Frequently_Asked_Questions

https://www.thomas-krenn.com/en/wiki/Ext4_Filesystem

https://access.redhat.com/labs/psb/versions/kernel-3.10.0-327.18.2.el7/Documentation/filesystems/ext4.txt

https://access.redhat.com/labs/psb/versions/kernel-3.10.0-327.18.2.el7/fs/ext4/ext4.h