Fix partial block encryption in cfb and ofb for s390x

Fix partial block encryption in cfb and ofb for s390x

Use the number of processed bytes information (num) from the generic
cipher context for the partial block handling in cfb and ofb, instead
of keep this information in the s390x-specific part of the cipher
context. The information in the generic context is reset properly,
even if the context is re-initialized without resetting the key or iv.

Fixes: #23175
Signed-off-by: Holger Dengler <dengler@linux.ibm.com>