ext4: check s_log_groups_per_flex in online resize code
If groups_per_flex < 2, sbi->s_flex_groups[] doesn't get filled out,
and every other access to this first tests s_log_groups_per_flex;
same thing needs to happen in resize or we'll wander off into
a null pointer when doing an online resize of the file system.
Thanks to Christoph Biedl, who came up with the trivial testcase: