[S390] vmur: diag14 only works with buffers below 2GB

If memory buffers above 2GB are used, diagnose 14 raises a specification
exception. This fix ensures that buffer allocation is done below the 2GB
boundary.

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