Revert "Make device_busy/unbusy work w/o Giant held"

This reverts commit 08e781915363f98f4318a864b3b5a52bd99424c6.

Commit message was for a very old version of the patch. Will re-commit
with the right one since it's so bad. There's no locked versions of
it...that code was reworked to use refcnt APIs.

Noticed by: jhb, jtrc27
Sponsored by: Netflix