mm/filemap: unify dropbehind flag testing and clearing

The read and write side does this a bit differently, unify it such that
the _{read,write} helpers check the bit before locking, and the generic
handler is in charge of clearing the bit and invalidating, once under
the folio lock.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/20250527133255.452431-6-axboe@kernel.dk


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