xfs: don't assert fail on perag references on teardown

Not fatal, the assert is there to catch developer attention. I'm
seeing this occasionally during recoveryloop testing after a
shutdown, and I don't want this to stop an overnight recoveryloop
run as it is currently doing.

Convert the ASSERT to a XFS_IS_CORRUPT() check so it will dump a
corruption report into the log and cause a test failure that way,
but it won't stop the machine dead.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>