locking/percpu-rwsem: Remove the embedded rwsem

The filesystem freezer uses percpu-rwsem in a way that is effectively
write_non_owner() and achieves this with a few horrible hacks that
rely on the rwsem (!percpu) implementation.

When PREEMPT_RT replaces the rwsem implementation with a PI aware
variant this comes apart.

Remove the embedded rwsem and implement it using a waitqueue and an
atomic_t.

 - make readers_block an atomic, and use it, with the waitqueue
   for a blocking test-and-set write-side.

 - have the read-side wait for the 'lock' state to clear.

Have the waiters use FIFO queueing and mark them (reader/writer) with
a new WQ_FLAG. Use a custom wake_function to wake either a single
writer or all readers until a writer.

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Davidlohr Bueso <[email protected]>
Acked-by: Will Deacon <[email protected]>
Acked-by: Waiman Long <[email protected]>
Tested-by: Juri Lelli <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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