Transfer responsibility of setting the *_UNINIT flags to libext2fs

Transfer responsibility of setting the *_UNINIT flags to libext2fs

Mke2fs used to have special case, ugly code in
setup_lazy_bg/setup_uninit_bg flag which set the flags based on all
sorts of special cases.  Change it so that it is done in libext2fs,
and fix mke2fs to use alloc_stats functions which will take care of
clearing the *_UNINIT flags automatically as needed.

This is preparatory work to make the flex_bg allocation patch much
cleaner.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>