Set zfs_arc_free_target to v_free_target.

Page daemon output is now regulated by a PID controller with a setpoint
of v_free_target. Moreover, the page daemon now wakes up regularly
rather than waiting for a wakeup from another thread. This means that
the free page count is unlikely to drop below the old
zfs_arc_free_target value, and as a result the ARC was not readily
freeing pages under memory pressure. Address the immediate problem by
updating zfs_arc_free_target to match the page daemon's new behaviour.

Reported and tested by: truckman
Discussed with: jeff
X-MFC with: r329882
Differential Revision: https://reviews.freebsd.org/D14994