SUNRPC: Fix a Oops when destroying the RPCSEC_GSS credential cache

Commit 07d02a67 causes a use-after free in the RPCSEC_GSS credential
destroy code, because the call to get_rpccred() in gss_destroying_context()
will now always fail to increment the refcount.

While we could just replace the get_rpccred() with a refcount_set(), that
would have the unfortunate consequence of resurrecting a credential in
the credential cache for which we are in the process of destroying the
RPCSEC_GSS context. Rather than do this, we choose to make a copy that
is never added to the cache and use that to destroy the context.

Fixes: 07d02a67

 ("SUNRPC: Simplify lookup code")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>