fuse: convert fuse_writepages_fill() to use a folio for its tmp page

fuse: convert fuse_writepages_fill() to use a folio for its tmp page

To pave the way for refactoring out the shared logic in
fuse_writepages_fill() and fuse_writepage_locked(), this change converts
the temporary page in fuse_writepages_fill() to use the folio API.

This is similar to the change in commit e0887e095a80 ("fuse: Convert
fuse_writepage_locked to take a folio"), which converted the tmp page in
fuse_writepage_locked() to use the folio API.

inc_node_page_state() is intentionally preserved here instead of
converting to node_stat_add_folio() since it is updating the stat of the
underlying page and to better maintain API symmetry with
dec_node_page_stat() in fuse_writepage_finish_stat().

No functional changes added.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>