mm/filemap: unify read/write dropbehind naming

The read side is filemap_end_dropbehind_read(), while the write side
used folio_ as the prefix rather than filemap_. The read side makes more
sense, unify the naming such that the write side follows that.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/20250527133255.452431-5-axboe@kernel.dk


Signed-off-by: Christian Brauner <brauner@kernel.org>