percpu_ref: reduce memory footprint of percpu_ref in fast path

percpu_ref: reduce memory footprint of percpu_ref in fast path

'struct percpu_ref' is often embedded into one user structure, and the
instance is usually referenced in fast path, however actually only
'percpu_count_ptr' is needed in fast path.

So move other fields into one new structure of 'percpu_ref_data', and
allocate it dynamically via kzalloc(), then memory footprint of
'percpu_ref' in fast path is reduced a lot and becomes suitable to put
into hot cacheline of user structure.

Signed-off-by: Ming Lei <[email protected]>
Tested-by: Veronika Kabatova <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Cc: Sagi Grimberg <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Bart Van Assche <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>