mke2fs: make s_inodes_per_group >= 8 in ext2fs_initialize

mke2fs: make s_inodes_per_group >= 8 in ext2fs_initialize

current mkfs.ext4 fails if we tried with the following parameters:

	mkfs.ext4  -m 0 -N 16 -O ^has_journal,^resize_inode,^uninit_bg,extent,meta_bg -b 1024 /dev/sdb3

It will cause segfault, but it is caused by another issue.  See my
patch "mke2fs: Avoid segmentation fault in
ext2fs_alloc_generic_bmap". And with that patch, the mkfs.ext4 will
fail with the error: /dev/sdb3: Memory allocation failed while setting
up superblock

The reason is that in ext2fs_initialize, we align s_inodes_per_group
to 8, but fails to consider the case that s_inodes_per_group < 8.
So make at least 8 inodes for s_inodes_per_group.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>