[PATCH] splice: redo page lookup if add_to_page_cache() returns -EEXIST

This can happen quite easily, if several processes are trying to splice
the same file at the same time. It's not a failure, it just means someone
raced with us in allocating this file page. So just dump the allocated
page and relookup the original.

Signed-off-by: Jens Axboe <axboe@suse.de>