This patch changes iscsi_target_handle_csg_zero() to explicitly
reject login requests in SecurityNegotiation with a zero-length
payload, following the language in RFC-3720 Section 8.2:

  Whenever an iSCSI target gets a response whose keys, or their
  values, are not according to the step definition, it MUST answer
  with a Login reject with the "Initiator Error" or "Missing Parameter"
  status.

Previously when a zero-length login request in CSG=0 was received,
the target would send a login response with CSG=0 + T_BIT=0 asking
the initiator to complete authentication, and not fail the login
until MAX_LOGIN_PDUS was reached.  This change will now immediately
fail the login attempt with ISCSI_STATUS_CLS_INITIATOR_ERR status.
Reported-by: Tejas Vaykole <tejas.vaykole@calsoftinc.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>