OPENAFS-SA-2018-002 afs: prevent RXAFSCB_GetLock information leak

RXAFSCB_GetLock (cmdebug) does not correctly initialize its output.
This leaks kernel memory over the wire:

struct AFSDBLock
- up to 14 bytes for member name (16 - '<cellname>\0')

Initialize the buffer.

(cherry picked from commit b52eb11a08)

Change-Id: If84c5d9d805356cd56be77313149a931a948b4d5

src/afs/afs_callback.c

@@ -306,6 +306,7 @@ SRXAFSCB_GetLock(struct rx_call *a_call, afs_int32 a_index,
     XSTATS_START_CMTIME(AFS_STATS_CM_RPCIDX_GETLOCK);
 
     AFS_STATCNT(SRXAFSCB_GetLock);
+    memset(a_result, 0, sizeof(*a_result));
     nentries = sizeof(ltable) / sizeof(struct ltable);
     if (a_index < 0 || a_index >= nentries+afs_cellindex) {
 	/*