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.

Change-Id: Ie82f68c5eae76519fcf1491164133c1955ed9c08
Reviewed-on: http://gerrit.openafs.org/5060
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>