ovs-thread: Issue better diagnostics for locking uninitialized mutexes.

This makes the message issued refer to the file and line that called
ovs_mutex_lock(), instead of to the file and line *inside*
ovs_mutex_lock().

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>