LU-3259 clio: Revise read ahead implementation

LU-3259 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 <jinshan.xiong@intel.com>
Change-Id: Ic388e3a3f744ea5a8352cc8529e32a71073bddb3
Reviewed-on: http://review.whamcloud.com/10859
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>