iscsi-target: Fix uninitialized usage of cmd->pad_bytes

This patch fixes an uninitialized usage of cmd->pad_bytes inside of
iscsit_handle_text_cmd() introduced during a v4.1 change to use cmd
members instead of local pad_bytes variables.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>