xfs: allocate m_errortag early

xfs: allocate m_errortag early

Ensure the mount structure always has a valid m_errortag for debug
builds.  This removes the NULL checking from the runtime code, and
prepares for allowing to set errortags from mount.

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>