eth: fbnic: Account for page fragments when updating BDQ tail

FBNIC supports fixed size buffers of 4K. When PAGE_SIZE > 4K, we
fragment the page across multiple descriptors (FBNIC_BD_FRAG_COUNT).
When refilling the BDQ, the correct number of entries are populated,
but tail was only incremented by one. So on a system with 64K pages,
HW would get one descriptor refilled for every 16 we populate.

Additionally, we program the ring size in the HW when enabling the BDQ.
This was not accounting for page fragments, so on systems with 64K pages,
the HW used 1/16th of the ring.

Fixes: 0cb4c0a13723 ("eth: fbnic: Implement Rx queue alloc/start/stop/free")
Signed-off-by: Dimitri Daskalakis <daskald@meta.com>
Link: https://patch.msgid.link/20260324195123.3486219-2-dimitri.daskalakis1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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