protocol/client: prevent use-after-free of frame->root

protocol/client: prevent use-after-free of frame->root

A regression failure generated a coredump on the glusterfs-client side:

  (gdb) f 0
  #0  0x00007fba6cd76432 in client_submit_request (this=0x7fba68006fc0,
                  req=0x7fba6579aa70, frame=0x7fba5c0058cc,
                  prog=0x7fba6cfb53c0 <clnt3_3_fop_prog>, procnum=41,
                  cbkfn=0x7fba6cd9206d <client3_3_release_cbk>,
                  iobref=0x0, rsphdr=0x0, rsphdr_count=0,
                  rsp_payload=0x0, rsp_payload_count=0, rsp_iobref=0x0,
                  xdrproc=0x7fba79801075 <xdr_gfs3_release_req>) at
  /home/jenkins/root/workspace/rackspace-regression-2GB-triggered/xlators/protocol/client/src/client.c:324
  324                   frame->root->ngrps = ngroups;
  (gdb) l
  319                   gf_msg_debug (this->name, 0, "rpc_clnt_submit failed");
  320           }
  321
  322           if (!conf->send_gids) {
  323                   /* restore previous values */
  324                   frame->root->ngrps = ngroups;
  325                   if (ngroups <= SMALL_GROUP_COUNT)
  326                           frame->root->groups_small[0] = gid;
  327           }
  328
  (gdb) p *frame->root
  Cannot access memory at address 0x64185df000000000

After looking at this in more detail, the flow is like this:

  client_submit_request()
    |
    '- rpc_clnt_submit() // on line 314
         |
         '- cbkfn() // = client3_3_release_cbk
              |
              :- STACK_DESTROY (frame->root);
         .----'
    .----'
    |
    :- frame->root->ngrps = ngroups; // on line 324
    '

So, there is a use-after-free, and it is not needed to restore the
previous groups in frame->root.

Change-Id: I9e7d712183692ed92cfc2f75cd3c2781a9db20e2
BUG: 128128
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/12575
Reviewed-by: Dan Lambright <dlambrig@redhat.com>
Tested-by: NetBSD Build System <jenkins@build.gluster.org>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>