[bugfix] [rtai] fix cleanup of locks

rt_get_adr didn't increment usage count of the lock in RTAI's internal,
so I had to take care of that same way I did in POSIX back-end.  It
would have been easier if rt_get_adr_cnt existed in user-space.

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>