libafs: Rate-limit hard-mount waiting messages

libafs: Rate-limit hard-mount waiting messages

Limit how often we log "hard-mount waiting for XXX" messages. Without
this, it is possible for a client with hard-mounts enabled to spam the
kernel log rather excessively (in extreme cases this can even panic
the machine on at least some Linux).

To keep things simple, just log approximately one message per volume
per hard-mount interval.

Reviewed-on: http://gerrit.openafs.org/5060
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 530b5ecac51cc7ce61ccddd50868c632c4a47298)

Change-Id: I566aa3d411ff100ccc6afa9a5273fb84e6172dd0
Reviewed-on: http://gerrit.openafs.org/6347
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>