block/iscsi: fix double-free on BUSY or similar statuses

Commit 8c460269 ("iscsi: base all handling of check condition on
scsi_sense_to_errno", 2019-07-15) removed a "goto out" so that the
same coroutine is re-entered twice; once from iscsi_co_generic_cb,
once from the timer callback iscsi_retry_timer_expired.  This can
cause a crash.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1378
Reported-by: Grzegorz Zdanowski <https://gitlab.com/kiler129>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 5080152e)
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

block/iscsi.c

@@ -268,6 +268,7 @@ iscsi_co_generic_cb(struct iscsi_context *iscsi, int status,
                 timer_mod(&iTask->retry_timer,
                           qemu_clock_get_ms(QEMU_CLOCK_REALTIME) + retry_time);
                 iTask->do_retry = 1;
+                return;
             } else if (status == SCSI_STATUS_CHECK_CONDITION) {
                 int error = iscsi_translate_sense(&task->sense);
                 if (error == EAGAIN) {