ihandle: OPEN fdPs are not counted in ihP refcount

ihandle: OPEN fdPs are not counted in ihP refcount

Just add a comment explaining that an OPEN FdHandle_t does not count
against the ref count for its parent IHandle_t. Recently I've seen
some confusion about this when discussing ihandle internals, and this
should make this abundantly clear.

Reviewed-on: http://gerrit.openafs.org/5317
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 0f4dfaed6b25ae4282298cc2ba4908ce9f36f043)

Change-Id: Icd0d5b368ccc679967e14b2460f47c814598c797
Reviewed-on: http://gerrit.openafs.org/6310
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>