sbp-target: rename a variable to avoid name clash

'int login_id' shadows 'static atomic_t login_id'.
Seen as compilation warning on x86-32.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Acked-by: Chris Boot <bootc@bootc.net>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>