This should make vnode_pager_getpages path a bit shorter and clearer.
Also this should eliminate problems with partially valid pages.
Having this method opens room for future optimizations.
To do: try to satisfy other pages besides the required one taking into
account tradeofs between number of page faults, read throughput and read
latency. Also, eventually vop_putpages should be added too.
Reviewed by: kib, mm, pjd
MFC after: 3 weeks
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