s390/zcore: calculate real memory size using own get_mem_size function

The zcore device driver makes use of the global visible real_memory_size
variable with its odd semantics.
Since the zcore device driver already has code in place which calculates
the memory size at module load time, use that code to calculate the current
memory end value.

One user less of the odd real_memory_size variable.

Reviewed-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>