xfs: remove xfs_errortag_get

xfs_errortag_get is only called by xfs_errortag_attr_show, which does not
need to validate the error tag, because it can only be called on valid
error tags that had a sysfs attribute registered.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>