mke2fs/libext2fs: Fix lazy inode table initialization

mke2fs/libext2fs: Fix lazy inode table initialization

This fixes some bugs which I introduced recently while revamping the
uninit_bg code.  Since mke2fs is no longer calling
ext2fs_set_gdt_csum(), it's important that ext2fs_initialize()
correctly initialize bg_itable_unused for all block group descriptors.
In addition, mke2fs needs to zero out the the reserved inodes based on
the values of bg_itable_unused set by ext2fs_initialize().

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