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 ]

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/20230303172120.3800725-2-shikemeng@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>