ext4: factor out ext4_flex_groups_free()

Factor out ext4_flex_groups_free() and it can be used both in
__ext4_fill_super() and ext4_put_super().

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20230323140517.1070239-5-yanaijie@huawei.com


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