On reconnect case, the iscsi_tcp_connect tries to reuse
the fd number of old_iscsi. However, this fd could have been
already closed in previous iscsi_tcp_disconnect if
iscsi->fd == iscsi->old_iscsi->fd and the fd number
might have been allocated to some other caller, in this
case the fd reuse in iscsi_tcp_connect is not safe anymore.
Solve this by not closing the fd if iscsi and old_iscsi
share the same fd on reconnect to "really" reserve this
fd number.
Signed-off-by: Tianren Zhang <tianren@smartx.com>