s390/kdump: fix REGSET_VX_LOW vector register ELF notes

[ Upstream commit 3c8e5105e759e7b2d88ea8a85b1285e535bc7500 ]

The REGSET_VX_LOW ELF notes should contain the lower 64 bit halfes of the
first sixteen 128 bit vector registers. Unfortunately currently we copy
the upper halfes.

Fix this and correctly copy the lower halfes.

Fixes: a62bc073 ("s390/kdump: add support for vector extension")
Cc: stable@vger.kernel.org # 3.18+
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>