s390/dasd: add missing discipline function

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

s390/dasd: add missing discipline function

Fix crash with illegal operation exception in dasd_device_tasklet.

Commit b72949328869 ("s390/dasd: Prepare for additional path event handling")

renamed the verify_path function for ECKD but not for FBA and DIAG.

This leads to a panic when the path verification function is called for a

FBA or DIAG device.

Fix by defining a wrapper function for dasd_generic_verify_path().