common/quota: move _qsetup() helper to common code

It's already used in two tests and will be used in a third.

Cc: Eryu Guan <guan@eryu.me>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: fstests@vger.kernel.org
Suggested-by: Eryu Guan <guan@eryu.me>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
3 files changed