fuse: only reuse auxiliary request in fuse_writepage_in_flight()

Don't reuse the queued request, even if it only contains a single page.
This is needed because previous locking changes (spliting out
fiq->waitq.lock from fc->lock) broke the assumption that request will
remain in FR_PENDING at least until the new page contents are copied.

This fix removes a slight optimization for a rare corner case, so we really
shoudln't care.

Reported-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Fixes: fd22d62e ("fuse: no fc->lock for iqueue parts")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>