s390/dasd: remove superfluous virt_to_phys() conversion

Only the last 12 bits of virtual / physical addresses are used when masking
with IDA_BLOCK_SIZE - 1. Given that the bits are the same regardless of
virtual or physical address, remove the virtual to physical address
conversion.

Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>