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.

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>
(cherry picked from commit 37c20528df26a43ec81b8e0f7658f1516d3c1b39)

Change-Id: I7e78989c313269388808c26d521f4075c0749cd4
Reviewed-on: http://gerrit.openafs.org/3638
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>