Fix inode table allocation with flexbg

Fix inode table allocation with flexbg

Disordered inode tables may appear when inode_blocks_per_group is lesser
or equal to the number of groups in a flex group.

This bug can be reproduced with:
	mkfs.ext4 -t ext4dev -G512 70G

In that case, you can see with dump2fs that inode tables for groups 510
and 511 are placed just after group 51's inode table instead of being
placed after group 509's inode table.

Signed-off-by: Frederic Bohe <frederic.bohe@bull.net>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>