fd: Move from using device_busy to a refcount

Use refcounting to delay the detach rather than device_busy and/or
device_unbusy. fd/fdc is one of the few consumers of device_busy in the
tree for that, and it's not a good fit. Also, nothing is waking 'fd' and
other drivers don't loop like this. Return EBUSY if we still have active
users.

Sponsored by: Netflix
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D31830