Clean-up: ust-consumer: fix -Wshadow issues in lttng_ustconsumer_recv_cmd

Same rationale and idea as previous patch, but for UST.

To differentiante the top-level `channel` variable, which owns a
channel, and the non-owning `channel` variables used when looking up
existing channels, the latter are renamed `found_channel`.

Change-Id: I6bf94be8d4368b5716756f4e12bae17c3e0d7cee
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>