fuse: writepages: roll back changes if request not found

fuse_writepage_in_flight() returns false if it fails to find request with
given index in fi->writepages.  Then the caller proceeds with populating
data->orig_pages[] and incrementing req->num_pages.  Hence,
fuse_writepage_in_flight() must revert changes it made in request before
returning false.

Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>