The patch http://review.whamcloud.com/11088 that was landed under this bug to remove the "extents" mount option was reverted via http://review.whamcloud.com/11267 because it was causing test failures in conf-sanity test_32a and test_32b. It seems these test images contain mount options with the "extents" string in their configurations, so it isn't just a matter of removing any current users of this option, we also need to handle upgrades from old filesystems (which is exactly what conf-sanity.sh test_32* is about).
I think what is needed is to check what version of Lustre stopped setting the "extents" mount option explicitly, and then we can make a decision based on when we deprecate support for that version and/or require a --writeconf before mounting. It would also be helpful to document this information in the header for these patches so this is easy to find in the future.
Patches for FC19 landed to Master.