ext4: set goal start correctly in ext4_mb_normalize_request

In the Linux kernel, the following vulnerability has been resolved:

ext4: set goal start correctly in ext4_mb_normalize_request

We need to set ac_g_ex to notify the goal start used in

ext4_mb_find_by_goal. Set ac_g_ex instead of ac_f_ex in

ext4_mb_normalize_request.

Besides we should assure goal start is in range [first_data_block,

blocks_count) as ext4_mb_initialize_context does.

[ Added a check to make sure size is less than ar->pright; otherwise

we could end up passing an underflowed value of ar->pright - size to

ext4_get_group_no_and_offset(), which will trigger a BUG_ON later on.

- TYT ]