eth fbnic: Add debugfs hooks for tx/rx rings

eth fbnic: Add debugfs hooks for tx/rx rings

Add debugfs hooks to display tx/rx rings for each napi
vector.

Note that the cloning mechanism in fbnic_ethtool.c for configuration
changes protects against concurrency issues with simultaneous config
changes along with debugs ring accesses.

The configuration switch builds up the new configuration offline,
takes the current config down, which removes the debugfs nv files, and
switches to the new configuration.   The new configuration is brought
up which brings the debugfs files back on top of the new configuration
rings.

The interaction with fbnic_queue_stop() and fbnic_queue_start() will
similarly delete and add the files for the indicated vector.

Signed-off-by: Mike Marciniszyn (Meta) <mike.marciniszyn@gmail.com>
Link: https://patch.msgid.link/20260127200644.11640-3-mike.marciniszyn@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>