Clear enclosure sysfs path from VDEV label when sysfs path isn't present

This clears vdev_enc_sysfs_path from the label if the VDEV's
/sys/class/block/<dev>/device/enclosure_device path isn't present.

This is important in the case where a disk that is labeled with
vdev_enc_sysfs_path is pulled out and put into another enclosure.
In that case, it's possible that the old sysfs path would be used to
turn on the fault LED for the disk's old slot postion, assuming the
new slot didn't have a LED sysfs entry.

Reviewed-by: Don Brady <don.brady@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #5524
Closes #5773