scsi_dh: Use the correct module name when loading device handler

This fixes a bug in recent kernels which results in failure to boot
on systems that have multipath SCSI disks.  I observed this failure
on a POWER8 server where all the disks are multipath SCSI disks.
The symptoms are several messages like this on the console:

[    3.018700] device-mapper: table: 253:0: multipath: error attaching hardware handler
[    3.018828] device-mapper: ioctl: error adding target to table

and the system does not find its disks, and therefore fails to boot.

Bisection revealed that the bug was introduced in commit 566079c8,
"dm-mpath, scsi_dh: request scsi_dh modules in scsi_dh, not dm-mpath".
The specific reason for the failure is that where we previously loaded
the "scsi_dh_alua" module, we are now trying to load the "alua" module,
which doesn't exist.

To fix this, we change the request_module call in scsi_dh_lookup()
to prepend "scsi_dh_" to the name, just like the old code in
drivers/md/dm-mpath.c:parse_hw_handler() used to do.

[jejb: also fixes issue spotted by Sasha Levin that formatting
characters could be passed in via sysfs and cause issues with
request_module()]

Fixes: 566079c8
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Cc: Sasha Levin <sasha.levin@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>