In range cyclic mode, writepages() should first write dirty pages
in range [writeback_index, (pgoff_t)-1], then write pages in range
[0, writeback_index -1]. Besides, if writepages() encounters a page
that beyond EOF, it should restart from the beginning.
Signed-off-by: N"Yan, Zheng" <zyan@redhat.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>