staging: lustre: clio: Revise read ahead implementation

In this implementation, read ahead will hold the underlying DLM lock
to add read ahead pages. A new cl_io operation cio_read_ahead() is
added for this purpose. It takes parameter cl_read_ahead{} so that
each layer can adjust it by their own requirements. For example, at
OSC layer, it will make sure the read ahead region is covered by a
LDLM lock; at the LOV layer, it will make sure that the region won't
cross stripe boundary.

Legacy callback cpo_is_under_lock() is removed.

Signed-off-by: Jinshan Xiong <[email protected]>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3259
Reviewed-on: http://review.whamcloud.com/10859
Reviewed-by: John L. Hammond <[email protected]>
Reviewed-by: Bobi Jam <[email protected]>
Reviewed-by: Oleg Drokin <[email protected]>
Signed-off-by: James Simmons <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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