rx: conn->callNumber protected by conn_call_lock

rx: conn->callNumber protected by conn_call_lock

The conn->callNumber array should be protected by the conn_call_lock
since the conn_call_lock is what protects the binding of calls to
connection channels.

Change-Id: I9f9b4e8f90d1e4ebbc4429af286358807784d84f
Reviewed-on: http://gerrit.openafs.org/6629
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>