target/tcm_fc: Fix use-after-free of ft_tpg

commit 2c42be2dd4f6586728dba5c4e197afd5cfaded78 upstream.

ft_del_tpg checks tpg->tport is set before unlinking the tpg from the
tport when the tpg is being removed. Set this pointer in ft_tport_create,
or the unlinking won't happen in ft_del_tpg and tport->tpg will reference
a deleted object.

This patch sets tpg->tport in ft_tport_create, because that's what
ft_del_tpg checks, and is the only way to get back to the tport to
clear tport->tpg.

The bug was occuring when:

- lport created, tport (our per-lport, per-provider context) is
  allocated.
  tport->tpg = NULL
- tpg created
- a PRLI is received. ft_tport_create is called, tpg is found and
  tport->tpg is set
- tpg removed. ft_tpg is freed in ft_del_tpg. Since tpg->tport was not
  set, tport->tpg is not cleared and points at freed memory
- Future calls to ft_tport_create return tport via first conditional,
  instead of searching for new tpg by calling ft_lport_find_tpg.
  tport->tpg is still invalid, and will access freed memory.

see https://bugzilla.redhat.com/show_bug.cgi?id=1071340



Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>