do not reconnect if reconnect is already defered

If the amount of reconnects is limited with iscsi_set_reconnect_max_retries()
it might happen that iscsi_reconnect is called while there is already a deferred
reconnect.

Signed-off-by: Peter Lieven <pl@kamp.de>
fix leak of iscsi_context in iscsi_reconnect

in case the maximum number of reconnects is limited with
iscsi_set_reconnect_max_retries() the an iscsi_context
is leaked if the limit is exhausted.

Signed-off-by: Peter Lieven <pl@kamp.de>