ipc_lock — Lock an ipc structure without rw_mutex held
IPC identifier set
ipc id to look for
Look for an id in the ipc ids idr and lock the associated ipc object.
The ipc object is locked on exit.
This is the routine that should be called when the rw_mutex is not already held, i.e. idr tree not protected: it protects the idr tree in read mode during the idr_find.