eth: fbnic: support ring size configuration

Support ethtool -g / -G. Leverage the code added for -l / -L
to alloc / stop / start / free.

Check parameters against HW min/max but also our own min/max.
Min HW queue is 16 entries, we can't deal with TWQs that small
because of the queue waking logic. Add similar contraint on RCQ
for symmetry.

We need 3 sizes on Rx, as the NIC does header-data split two separate
buffer pools:
  (1) head page ring    - how many empty pages we post for headers
  (2) payload page ring - how many empty pages we post for payloads
  (3) completion ring   - where NIC produces the Rx descriptors

Acked-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250306145150.1757263-4-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>