xen-blkback: add a parameter for disabling of persistent grants

Persistent grants feature provides high scalability.  On some small
systems, however, it could incur data copy overheads[1] and thus it is
required to be disabled.  But, there is no option to disable it.  For
the reason, this commit adds a module parameter for disabling of the
feature.

[1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability

Signed-off-by: Anthony Liguori <[email protected]>
Signed-off-by: SeongJae Park <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Acked-by: Roger Pau Monné <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Boris Ostrovsky <[email protected]>

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