dm mpath: Add missing dm_put_device when failing to get scsi dh name

In the Linux kernel, the following vulnerability has been resolved:

dm mpath: Add missing dm_put_device when failing to get scsi dh name

When commit fd81bc5cca8f ("scsi: device_handler: Return error pointer in

scsi_dh_attached_handler_name()") added code to fail parsing the path if

scsi_dh_attached_handler_name() failed with -ENOMEM, it didn't clean up

the reference to the path device that had just been taken. Fix this, and

steamline the error paths of parse_path() a little.