s390/diag: add missing virt_to_phys() translation to diag14()

diag14() is currently only used by the vmur device driver.  The third
parameter, called subcommand, determines the type of the first
parameter. For some subcommands the value of the first parameter is an
address to a memory buffer and needs virtual to physical address
conversion. Other subcommands interpret the first parameter is an
integer.
This doesn't fix a bug since virtual and physical addresses
are currently the same.

Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>