iscsi-target: use kstrdup() for iscsi_param

The kmalloc() + strlen() + memcpy() block is what kstrdup() does as
well.  While here I also removed the "to NULL assignment" of pointers
which are fed to kfree or thrown away anyway.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>