zfs: bring per_txg_dirty_frees_percent back to 30
This cherry-picks upstream eb9bec0a5d19abf9404f52081424fbb814e6188a
The current value causes significant artificial slowdown during mass parallel file removal, which can be observed both on FreeBSD and Linux when running real workloads. Sample results from Linux doing make -j 96 clean after an allyesconfig modules build: before: 4.14s user 6.79s system 48% cpu 22.631 total after: 4.17s user 6.44s system 153% cpu 6.927 total FreeBSD results in the ticket.