Fix unable to export zpool without nfs-utils

Don't return error in nfs_disable_share when nfs is not available, since
it wouldn't have been able to share in the first place.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes #13534
Closes #13800