target/iscsi: cleanup some allocation style issues

We can use kcalloc() here instead of kzalloc().  It's better style and
it has overflow checking built in.

Also -ENOMEM is the correct error code for allocation errors.  -1 means
-EPERM.  None of the callers preserve the error codes so it doesn't
matter except as a cleanup.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>