io_uring/rsrc: optimise io_rsrc_put allocation

Every io_rsrc_node keeps a list of items to put, and all entries are
kmalloc()'ed. However, it's quite often to queue up only one entry per
node, so let's add an inline entry there to avoid extra allocations.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c482c1c652c45c85ac52e67c974bc758a50fed5f.1680576071.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>