fuse: use fuse_range_is_writeback() instead of iterating pages

fuse_send_readpages() waits for writeback on each page.  This can be
replaced by a single call to fuse_range_is_writeback().

[SzM: split this off from "fuse: convert readahead to use folios"]

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>