s390/xpram: don't modify module parameters

During parsing of the sizes array the pointer to the particular
string is lost. Keep it by using an extra pointer to store the end
position of the parsed string. Keeping these parameters accessible
can be helpful for debugging purposes and for userspace reading
the parameters at runtime via sysfs. Also this will ensure that the
memory is freed at module unload time.

Reported-by: Michael Veigel <veigel@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>