ihandle: Actually assert active fdPs are not AVAIL

ihandle: Actually assert active fdPs are not AVAIL

FdHandle_t's that are on the linked list for an associated IHandle_t
should not be in the state FD_HANDLE_AVAIL. For the non-PIO case, we
assert that this is the case in ih_open (since we assert that if the
FdHandle_t is not in INUSE state, then it must be in OPEN state).
However, for the PIO case, we were just skipping over any FdHandle_t's
that were in the AVAIL state. These should never exist while on that
linked list, so assert for the PIO case, as well.

In the absence of bugs, there is no functional change here, but it
perhaps makes the ih_open loop easier to understand.

Change-Id: I4e3b3319ae14b7e68a87f5fe172419f5ebf9d2c9
Reviewed-on: http://gerrit.openafs.org/5307
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>