xfs: simulate per-AG reservations being critically low

Create an error injection point that enables us to simulate being
critically low on per-AG block reservations.  This should enable us to
simulate this specific ENOSPC condition so that we can test falling back
to a regular file copy.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>