Fix a race in which the RPC client is shutting down while the
gss daemon is processing a downcall. If the RPC client manages to
shut down before the gss daemon is done, then the struct gss_auth
used in gss_release_msg() may have already been freed.

Link: http://lkml.kernel.org/r/1392494917.71728.YahooMailNeo@web140002.mail.bf1.yahoo.comReported-by: NJohn <da_audiophile@yahoo.com>
Reported-by: NBorislav Petkov <bp@alien8.de>
Cc: stable@vger.kernel.org # 3.12+
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>