NFSv4: test SECINFO RPC_AUTH_GSS pseudoflavors for support

Fix nfs4_negotiate_security to create an rpc_clnt used to test each SECINFO
returned pseudoflavor. Check credential creation  (and gss_context creation)
which is important for RPC_AUTH_GSS pseudoflavors which can fail for multiple
reasons including mis-configuration.

Don't call nfs4_negotiate in nfs4_submount as it was just called by
nfs4_proc_lookup_mountpoint (nfs4_proc_lookup_common)

Signed-off-by: Andy Adamson <andros@netapp.com>
[Trond: fix corrupt return value from nfs_find_best_sec()]
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/nfs4_fs.h b/fs/nfs/nfs4_fs.h
index f63cb87..ba2affa 100644
--- a/fs/nfs/nfs4_fs.h
+++ b/fs/nfs/nfs4_fs.h
@@ -230,7 +230,7 @@
 extern struct file_system_type nfs4_fs_type;
 
 /* nfs4namespace.c */
-struct rpc_clnt *nfs4_create_sec_client(struct rpc_clnt *, struct inode *, struct qstr *);
+struct rpc_clnt *nfs4_negotiate_security(struct rpc_clnt *, struct inode *, struct qstr *);
 struct vfsmount *nfs4_submount(struct nfs_server *, struct dentry *,
 			       struct nfs_fh *, struct nfs_fattr *);
 int nfs4_replace_transport(struct nfs_server *server,