Reduce async_destroy_001_pos memory requirements

The number of blocks which can be freed per TXG is controlled
by the zfs_free_max_blocks module option (defaults to 100,000).
Both speed up this test case and reduce the memory requirements
by only creating 4 TXGs worth of blocks to be freed.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #5479
Closes #6192