Disks can go missing until a reboot is done in some cases.

This is due to the DevHandle not being released, which causes the Firmware to
not allow that disk to be re-added.

Reviewed by: ken, scottl, ambrisko, asomers
Approved by: ken, scottl, ambrisko
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6102