Revert "Revert "ext4: make mb_optimize_scan performance mount option work with extents""

This reverts commit e05a335a.

The revert should no longer be necessary after upstream:
ext4: avoid unnecessary spreading of allocations among groups

See: https://github.com/raspberrypi/linux/issues/5097


Signed-off-by: Dom Cobley <popcornmix@gmail.com>