BUG/MINOR: cache: Fix loop on cache entries in "show cache"

BUG/MINOR: cache: Fix loop on cache entries in "show cache"

A regression was introduced in the commit da91842b6 ("BUG/MEDIUM: cache/cli:
make "show cache" thread-safe"). When cli_io_handler_show_cache() is called,
only one node is retrieved and is used to fill the output buffer in loop.
Once set, the "node" variable is never renewed. At the end, all nodes are
dumped but each one is duplicated several time into the output buffer.

This patch must be backported everywhere the above commit is. It means only
to 2.5 and 2.4.