The vast majority of the busy/unbusy users in the tree don't acquire Giant
before calling device_busy/unbusy. However, if multiple threads are opening a
file, say, that causes the device to busy/unbusy, then we can race to the root
marking things busy. Create a new device_busy_locked and device_unbusy_locked
that are the current implemntations of device_busy and device_unbusy. Make
device_busy and unbusy acquire Giant before calling the _locked versrions. Since
we never sleep in the busy/unbusy path, Giant's single threaded semantics
suffice to keep this safe.

Sponsored by:		Netflix
Reviewed by:		hselasky, jhb
Differential Revision:	https://reviews.freebsd.org/D26284