tipc: convert config_lock from spinlock to mutex

As the configuration server is now running under process context,
it's unnecessary for us to have a spinlock serializing the TIPC
configuration process. Instead, we replace it with a mutex lock,
which gives us more freedom. For instance, we can now call
pre-emptable functions within the protected area.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>