xfs: abort consistently on dquot flush failure

The dquot flush handler effectively aborts the dquot flush if the
filesystem is already shut down, but doesn't actually shut down if
the flush fails. Update xfs_qm_dqflush() to consistently abort the
dquot flush and shutdown the fs if the flush fails with an
unexpected error.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>