ztest_deadman_thread(): Account for -T option.

Instead of hardcoding the grace period to 300 seconds, use the
zo_time field, so it can be run for longer if desired, without
tripping the deadman thread.
Strengthen dbuf_free_range() by asserting dn_struct_rwlock.

Update one caller of the dbuf_rm_spill() API so that this can be done.

Submitted by:	Justin Gibbs <justing@spectralogic.com>