zpool command complains about /etc/exports.d

If the /etc/exports.d directory does not exist, then we should only
create it when we're performing an action which already requires root
privileges.

This commit moves the directory creation to the enable/disable code
path which ensures that we have the appropriate privileges.

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Wilson <gwilson@delphix.com>
Closes #10785
Closes #10934