[IB] uverbs: Close some exploitable races

Al Viro pointed out that the current IB userspace verbs interface
allows userspace to cause mischief by closing file descriptors before
we're ready, or issuing the same command twice at the same time.  This
patch closes those races, and fixes other obvious problems such as a
module reference leak.

Some other interface bogosities will require an ABI change to fix
properly, so I'm deferring those fixes until 2.6.15.

Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/core/uverbs.h

@@ -69,6 +69,7 @@ struct ib_uverbs_event_file {
 
 struct ib_uverbs_file {
 	struct kref				ref;
+	struct semaphore			mutex;
 	struct ib_uverbs_device		       *device;
 	struct ib_ucontext		       *ucontext;
 	struct ib_event_handler			event_handler;