Only automatically mount a clone when 'canmount == on'.

According to the man page, "When the noauto option is set, a dataset
can only be mounted and unmounted explicitly. The dataset is not
mounted automatically when the dataset is created or imported ...."

When cloning a dataset the canmount property was not being honored.
This patch adds the required check to achieve the behavior described
in the man page.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2241