zfs: call vnode_pager_setsize() to set vnode size in zfs_write()

This is what original frebsd zfs code has and it was changed by
c218cd1ed55f6903f3 because OSv lacked this function, but the function
was introduce in a063ab93059e123fc9.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>