Bring per_txg_dirty_frees_percent back to 30

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.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes #13932
Closes #13938