zfs: add vop_getpages method implementation

zfs: add vop_getpages method implementation

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