RPCSEC_GSS: fix crash on destroying gss auth

This fixes a regression since  eb6dc19d8e72ce3a957af5511d20c0db0a8bd007
"RPCSEC_GSS: Share all credential caches on a per-transport basis" which
could cause an occasional oops in the nfsd code (see below).

The problem was that an auth was left referencing a client that had been
freed.  To avoid this we need to ensure that auths are shared only
between descendants of a common client; the fact that a clone of an
rpc_client takes a reference on its parent then ensures that the parent
client will last as long as the auth.

Also add a comment explaining what I think was the intention of this
code.

  general protection fault: 0000 [#1] PREEMPT SMP
  Modules linked in: rpcsec_gss_krb5 nfsd auth_rpcgss oid_registry nfs_acl lockd sunrpc
  CPU: 3 PID: 4071 Comm: kworker/u8:2 Not tainted 3.11.0-rc2-00182-g025145f #1665
  Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
  Workqueue: nfsd4_callbacks nfsd4_do_callback_rpc [nfsd]
  task: ffff88003e206080 ti: ffff88003c384000 task.ti: ffff88003c384000
  RIP: 0010:[<ffffffffa00001f3>]  [<ffffffffa00001f3>] rpc_net_ns+0x53/0x70 [sunrpc]
  RSP: 0000:ffff88003c385ab8  EFLAGS: 00010246
  RAX: 6b6b6b6b6b6b6b6b RBX: ffff88003af9a800 RCX: 0000000000000002
  RDX: ffffffffa00001a5 RSI: 0000000000000001 RDI: ffffffff81e284e0
  RBP: ffff88003c385ad8 R08: 0000000000000001 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000015 R12: ffff88003c990840
  R13: ffff88003c990878 R14: ffff88003c385ba8 R15: ffff88003e206080
  FS:  0000000000000000(0000) GS:ffff88003fd80000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
  CR2: 00007fcdf737e000 CR3: 000000003ad2b000 CR4: 00000000000006e0
  Stack:
   ffffffffa00001a5 0000000000000006 0000000000000006 ffff88003af9a800
   ffff88003c385b08 ffffffffa00d52a4 ffff88003c385ba8 ffff88003c751bd8
   ffff88003c751bc0 ffff88003e113600 ffff88003c385b18 ffffffffa00d530c
  Call Trace:
   [<ffffffffa00001a5>] ? rpc_net_ns+0x5/0x70 [sunrpc]
   [<ffffffffa00d52a4>] __gss_pipe_release+0x54/0x90 [auth_rpcgss]
   [<ffffffffa00d530c>] gss_pipe_free+0x2c/0x30 [auth_rpcgss]
   [<ffffffffa00d678b>] gss_destroy+0x9b/0xf0 [auth_rpcgss]
   [<ffffffffa000de63>] rpcauth_release+0x23/0x30 [sunrpc]
   [<ffffffffa0001e81>] rpc_release_client+0x51/0xb0 [sunrpc]
   [<ffffffffa00020d5>] rpc_shutdown_client+0xe5/0x170 [sunrpc]
   [<ffffffff81098a14>] ? cpuacct_charge+0xa4/0xb0
   [<ffffffff81098975>] ? cpuacct_charge+0x5/0xb0
   [<ffffffffa019556f>] nfsd4_process_cb_update.isra.17+0x2f/0x210 [nfsd]
   [<ffffffff819a4ac0>] ? _raw_spin_unlock_irq+0x30/0x60
   [<ffffffff819a4acb>] ? _raw_spin_unlock_irq+0x3b/0x60
   [<ffffffff810703ab>] ? process_one_work+0x15b/0x510
   [<ffffffffa01957dd>] nfsd4_do_callback_rpc+0x8d/0xa0 [nfsd]
   [<ffffffff8107041e>] process_one_work+0x1ce/0x510
   [<ffffffff810703ab>] ? process_one_work+0x15b/0x510
   [<ffffffff810712ab>] worker_thread+0x11b/0x370
   [<ffffffff81071190>] ? manage_workers.isra.24+0x2b0/0x2b0
   [<ffffffff8107854b>] kthread+0xdb/0xe0
   [<ffffffff819a4ac0>] ? _raw_spin_unlock_irq+0x30/0x60
   [<ffffffff81078470>] ? __init_kthread_worker+0x70/0x70
   [<ffffffff819ac7dc>] ret_from_fork+0x7c/0xb0
   [<ffffffff81078470>] ? __init_kthread_worker+0x70/0x70
  Code: a5 01 00 a0 31 d2 31 f6 48 c7 c7 e0 84 e2 81 e8 f4 91 0a e1 48 8b 43 60 48 c7 c2 a5 01 00 a0 be 01 00 00 00 48 c7 c7 e0 84 e2 81 <48> 8b 98 10 07 00 00 e8 91 8f 0a e1 e8
  +3c 4e 07 e1 48 83 c4 18
  RIP  [<ffffffffa00001f3>] rpc_net_ns+0x53/0x70 [sunrpc]
   RSP <ffff88003c385ab8>

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Merge tag 'nfs-for-3.12-3' of git://git.linux-nfs.org/projects/trondmy/linux-nfs

Pull NFS client bugfix from Trond Myklebust:
 "Fix a regression due to incorrect sharing of gss auth caches"

* tag 'nfs-for-3.12-3' of git://git.linux-nfs.org/projects/trondmy/linux-nfs:
  RPCSEC_GSS: fix crash on destroying gss auth