eth: fbnic: Use shinfo to track frags state on Rx

eth: fbnic: Use shinfo to track frags state on Rx

Remove local fields that track frags state and instead store this
information directly in the shinfo struct. This change is necessary
because the current implementation can lead to inaccuracies in certain
scenarios, such as when using XDP multi-buff support. Specifically, the
XDP program may update nr_frags without updating the local variables,
resulting in an inconsistent state.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Link: https://patch.msgid.link/20250813221319.3367670-4-mohsin.bashr@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>