aio-posix: simplify FDMonOps->update() prototype

aio-posix: simplify FDMonOps->update() prototype

The AioHandler *node, bool is_new arguments are more complicated to
think about than simply being given AioHandler *old_node, AioHandler
*new_node.

Furthermore, the new Linux io_uring file descriptor monitoring mechanism
added by the new patch requires access to both the old and the new
nodes.  Make this change now in preparation.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Message-Id: <20200305170806.1313245[email protected]>