ext4: avoid s_mb_prefetch to be zero in individual scenarios

Commit cfd732377221 ("ext4: add prefetching for block allocation
bitmaps") introduced block bitmap prefetch, and expects to read block
bitmaps of flex_bg through an IO.  However, it seems to ignore the
value range of s_log_groups_per_flex.  In the scenario where the value
of s_log_groups_per_flex is greater than 27, s_mb_prefetch or
s_mb_prefetch_limit will overflow, cause a divide zero exception.

In addition, the logic of calculating nr is also flawed, because the
size of flexbg is fixed during a single mount, but s_mb_prefetch can
be modified, which causes nr to fail to meet the value condition of
[1, flexbg_size].

To solve this problem, we need to set the upper limit of
s_mb_prefetch.  Since we expect to load block bitmaps of a flex_bg
through an IO, we can consider determining a reasonable upper limit
among the IO limit parameters.  After consideration, we chose
BLK_MAX_SEGMENT_SIZE.  This is a good choice to solve divide zero
problem and avoiding performance degradation.

[ Some minor code simplifications to make the changes easy to follow -- TYT ]

Reported-by: Tosk Robot <[email protected]>
Signed-off-by: Chunguang Xu <[email protected]>
Reviewed-by: Samuel Liao <[email protected]>
Reviewed-by: Andreas Dilger <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

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