iscsi-target: Always send a response before terminating iSCSI connection

commit 1c5c12c666fda27c7c494b34934a0a0631a48130 upstream.

There are some cases, for example when the initiator sends an
out-of-bounds ErrorRecoveryLevel value, where the iSCSI target
terminates the connection without sending back any error.  Audit the
login path and add appropriate iscsit_tx_login_rsp() calls to make
sure this doesn't happen.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>