Zero rx_multi_lock before initializing it

Zero rx_multi_lock before initializing it

FreeBSD's kernel debugging facilities will panic if it detects
an attempt to re-initialize an already-initialized lock, as detected
through some particular bits being nonzero.  Zero everything out
before starting, to prevent this panic.

Change-Id: I7f3abae978d1cbe7eb4908d90c52a0dc46b81e08
Reviewed-on: http://gerrit.openafs.org/3550
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>