reiserfs: convert writepage to use a folio

Convert the incoming page to a folio and then use it throughout the
writeback path.  This definitely isn't enough to support large folios, but
I don't expect reiserfs to gain support for those before it is removed.

Link: https://lkml.kernel.org/r/20231016201114.1928083-23-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>