tcm_fc: Fix crash seen with aborts and large reads

This patch fixes a crash seen when large reads have their exchange
aborted by either timing out or being reset. Because the exchange
abort results in the seq pointer being set to NULL, because the
sequence is no longer valid, it must not be dereferenced. This
patch changes the function ft_get_task_tag to return ~0 if it is
unable to get the tag for this reason. Because the get_task_tag
interface provides no means of returning an error, this seems
like the best way to fix this issue at the moment.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>