scsi_dh: don't try to load a device handler during async probing

Request_module gets really unhappy when called from async probing, so
revert to not auto load device handler modules during the SCSI bus
scan.  While autoloading would be really useful we never did this
until 4.3-rc and it turns out that functionality doesn't actually
work.

Fixes: 566079 ("dm-mpath, scsi_dh: request scsi_dh modules in scsi_dh, not dm-mpath")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Paul Mackerras <paulus@ozlabs.org>
Tested-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: James Bottomley <JBottomley@Odin.com>