s390/zcore: Fix HSA copy length for last block

Currently always one page is copied to a user buffer for the last
HSA block in memcpy_hsa(). Now the correct length is used.

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