rtmutex: Add a special case for ww-mutex handling.

The lockdep selftest for ww-mutex assumes in a few cases the
ww_ctx->contending_lock assignment via __ww_mutex_check_kill() which
does not happen if the rtmutex detects the deadlock early.

The testcase passes if the deadlock handling here is removed. This means
that it will work if multiple threads/tasks are involved and not just a
single one.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>