fuse: call fuse_send_readpages explicitly from fuse_readahead

fuse: call fuse_send_readpages explicitly from fuse_readahead

Move the call to fuse_send_readpages from the iomap ->submit_read method
to the fuse readahead implementation.

fuse_read_folio() does not need to call fuse_send_readpages() because it
always does reads synchronously (the iomap->submit_read method for this
was a no-op since data->ia is always NULL for fuse_read_folio()).

This prepares for an iomap fix that will call ->submit_read after each
iomap.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260629121750.3392300-3-hch@lst.de
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>