futex: No need to check return value of debugfs_create functions

When calling debugfs functions, there is no need to ever check the return
value.  The function can work or not, but the code logic should never do
something different based on this.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Darren Hart (VMware) <dvhart@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190122152151.16139-40-gregkh@linuxfoundation.org