iscsi-target: Use list_first_entry() where appropriate

Instead of using the obfuscated pattern of

	list_for_each_entry(var, list, ...)
		break;

to set var to the first entry of a list, use the straightforward

	var = list_first_entry(list, ...);

Reported-by: Joern Engel <joern@logfs.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>