xfs: use WRITE_ONCE/READ_ONCE for m_errortag

xfs: use WRITE_ONCE/READ_ONCE for m_errortag

There is no synchronization for updating m_errortag, which is fine as
it's just a debug tool.  It would still be nice to fully avoid the
theoretical case of torn values, so use WRITE_ONCE and READ_ONCE to
access the members.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>