Revert "rwsem-rt: Do not allow readers to nest"

Revert "rwsem-rt: Do not allow readers to nest"

This behaviour is required by cpufreq and its logic is "okay": It does a
read_lock followed by a try_read_lock.
Lockdep warns if one try a read_lock twice in -RT and vanilla so it
should be good. We still only allow multiple readers as long as it is in
the same process.

Cc: stable-rt@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>