[multipathd] fix offline devices reported as active

[multipathd] fix offline devices reported as active

multipathd(8) continues calling reinstate_path() in checkerloop(),
if multipathd(8) is started on the system which has multipath maps
including offline devices.
The reason is pp->checkfn() returns -1 for offline devices and
pp->state of these paths are 1.
(I'm using "echo offline > /sys/block/sd?/device/state" for the testing.)

As a result, multipathd(8) makes offline devices active state.
And this behavior also affects the no_path_retry feature, since
struct multipath.nr_active is incremented in reinstate_path().

The following patch fixes this problem.
If the return value from pp->checkfn is -1, the path isn't available.
So I think that we can ignore it.

Kiyoshi Ueda, NEC