scsi: libsas: factor out sas_check_fanout_expander_topo()

To be consistent with sas_check_edge_expander_topo(), factor out
sas_check_fanout_expander_topo(). And remove the comment since we are not
spilling over 80 colums now.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20230421093744.1583609-4-yanaijie@huawei.com


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>