xfs: create convenience wrappers for incore quota block reservations

Create a couple of convenience wrappers for creating and deleting quota
block reservations against future changes.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>