Limit zfs_dirty_data_max_max to 4G

Reinstate default 4G zfs_dirty_data_max_max limit.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6072
Closes #6081