scsi_transport_srp: Fix fast_io_fail_tmo=dev_loss_tmo=off behavior

If scsi_remove_host() is called while an rport is in the blocked state
then scsi_remove_host() will only finish if the rport is unblocked
from inside a timer function. Make sure that an rport only enters the
blocked state if a timer will be started that will unblock it. This
avoids that unloading the ib_srp kernel module after having
disconnected the initiator from the target system results in a
deadlock if both the fast_io_fail_tmo and dev_loss_tmo parameters have
been set to "off".

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: David Dillow <dave@thedillows.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>