Dear Ben and Dave,

I fixed my problem:

I was using the version 3.1.7 of xfs_repair and eventhough the inode 
problem was detected:

....
         - agno = 3
data fork in regular inode 3224608213 claims used block 335579196
         - agno = 4
....

The problem was never corrected. So under the advice of Ben Meyers, I 
compiled the latest version from git of the xfsprogs (3.1.10) and used 
the xfs_repair tool from that version.
The problem was then addressed:

....
         - agno = 3
data fork in regular inode 3224608213 claims used block 335579196
correcting nextents for inode 3224608213
correcting nextents for inode 3224608213, was 1 - counted 0
         - agno = 4
.....

And it did fix the problem.

I want to thank you again for your concern on my problem.

best,

Guillaume Anciaux