eth: fbnic: Add protection against pause storm

eth: fbnic: Add protection against pause storm

Add protection against TX pause storms. A pause storm occurs when a
device fails to send received packets up to the stack. When a pause
storm is detected (pause state persists beyond the configured timeout),
the device stops sending the pause frames and begins dropping packets
instead of back-pressuring.

The timeout is configurable via ethtool tunable (pfc-prevention-tout)
with a maximum value of 10485ms, and the default value of 500ms.

Once the device transitions to the storm-detected state, the service
task periodically attempts recovery, returning the device to normal
operation to handle any subsequent pause storm episodes.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Link: https://patch.msgid.link/20260302230149.1580195-4-mohsin.bashr@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>