s390/sclp: read sccb from mem based on provided length

s390/sclp: read sccb from mem based on provided length

The header contained within the SCCB passed to the SCLP service call
contains the actual length of the SCCB. Instead of allocating a static
4K size for the work sccb, let's allow for a variable size determined
by the value in the header. The proper checks are already in place to
ensure the SCCB length is sufficent to store a full response and that
the length does not cross any explicitly-set boundaries.

Signed-off-by: Collin Walling <walling@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20200915194416.107460-4-walling@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>