Disable parallel processing for 'zfs mount -l'

Currently, 'zfs mount -a' will always attempt to parallelize
work related to mounting as best it can. Unfortunately, when
the user passes the '-l' option to load keys, this causes
all threads to prompt the user for their keys at once,
resulting in a confusing and racy user experience. This patch
simply disables parallel mounting when using the '-l' flag.

Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8762
Closes #8811