devclass_alloc_unit: move "at" hint test to after device-in-use test

Only perform this expensive operation when the unit number is a
potential candidate (i.e. not already in use), thereby reducing device
scan time on systems with many devices, unit numbers, and drivers.

Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
X-NetApp-PR: #61
Differential Revision: https://reviews.freebsd.org/D31381