ext2fs: Add uninit_bg feature support.

From the linux tune2fs(8) manpage:
"Allow the kernel to initialize bitmaps and inode tables and keep a high
watermark for the unused inodes in a filesystem, to reduce e2fsck(8) time.
This first e2fsck run after enabling this feature will take the full time,
but subsequent e2fsck runs will take only a fraction of the original time,
depending on how full the file system is."

Submitted by: Fedor Uporov
Differential Revision: https://reviews.freebsd.org/D11211