posix-cpu-timers: Respect INFINITY for hard RTTIME limit

The RTIME limit expiry code does not check the hard RTTIME limit for
INFINITY, i.e. being disabled.  Add it.

While this could be considered an ABI breakage if something would depend on
this behaviour. Though it's highly unlikely to have an effect because
RLIM_INFINITY is at minimum INT_MAX and the RTTIME limit is in seconds, so
the timer would fire after ~68 years.

Adding this obvious correct limit check also allows further consolidation
of that code and is a prerequisite for cleaning up the 0 based checks and
the rlimit setter code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lkml.kernel.org/r/20190821192922.078293002@linutronix.de