Coverity noticed that the error handling code in the NFSv4 callback client
sets cb->cb_client to NULL, then calls rpc_shutdown_client with the NULL
pointer.

Coverity: #cid 1397
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NNeil Brown <neilb@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>