btrfs: remove btrfs_set_range_writeback()

btrfs: remove btrfs_set_range_writeback()

The function btrfs_set_range_writeback() was originally a callback for
metadata and data, to mark a range with writeback flag.

Then it was converted into a common function call for both metadata and
data.

From the very beginning, the function had been only called on a full page,
later converted to handle range inside a page.

But it never needed to handle multiple pages, and since commit
8189197425e7 ("btrfs: refactor __extent_writepage_io() to do
sector-by-sector submission") the function was only called on a
sector-by-sector basis.

This makes the function unnecessary, and can be converted to a simple
btrfs_folio_set_writeback() call instead.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>