FROMGIT: erofs: fix wrong primary bvec selection on deduplicated extents

When handling deduplicated compressed data, there can be multiple
decompressed extents pointing to the same compressed data in one shot.

In such cases, the bvecs which belong to the longest extent will be
selected as the primary bvecs for real decompressors to decode and the
other duplicated bvecs will be directly copied from the primary bvecs.

Previously, only relative offsets of the longest extent were checked to
decompress the primary bvecs.  On rare occasions, it can be incorrect
if there are several extents with the same start relative offset.
As a result, some short bvecs could be selected for decompression and
then cause data corruption.

For example, as Shijie Sun reported off-list, considering the following
extents of a file:
117:   903345..  915250 |   11905 :     385024..    389120 |    4096
...
119:   919729..  930323 |   10594 :     385024..    389120 |    4096
...
124:   968881..  980786 |   11905 :     385024..    389120 |    4096

The start relative offset is the same: 2225, but extent 119 (919729..
930323) is shorter than the others.

Let's restrict the bvec length in addition to the start offset if bvecs
are not full.

Reported-by: Shijie Sun <sunshijie@xiaomi.com>
Fixes: 5c2a6425 ("erofs: introduce partial-referenced pclusters")
Tested-by Shijie Sun <sunshijie@xiaomi.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230719065459.60083-1-hsiangkao@linux.alibaba.com
(cherry picked from commit 7d15c91a75aae55767f368e8abbabd7cedf4ec94
https://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs.git

 dev)
Bug: 293245292
Change-Id: Ic8ded9b2d3592ffd0863f4f0d2ac4ae6a1821a1b
Signed-off-by: sunshijie <sunshijie@xiaomi.corp-partner.google.com>