net: bnxt: Implement software USO

Implement bnxt_sw_udp_gso_xmit() using the core tso_dma_map API and
the pre-allocated TX inline buffer for per-segment headers.

The xmit path:
1. Calls tso_start() to initialize TSO state
2. Stack-allocates a tso_dma_map and calls tso_dma_map_init() to
   DMA-map the linear payload and all frags upfront.
3. For each segment:
   - Copies and patches headers via tso_build_hdr() into the
     pre-allocated tx_inline_buf (DMA-synced per segment)
   - Counts payload BDs via tso_dma_map_count()
   - Emits long BD (header) + ext BD + payload BDs
   - Payload BDs use tso_dma_map_next() which yields (dma_addr,
     chunk_len, mapping_len) tuples.

Header BDs set dma_unmap_len=0 since the inline buffer is pre-allocated
and unmapped only at ring teardown.

Completion state is updated by calling tso_dma_map_completion_save() for
the last segment.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20260408230607.2019402-8-joe@dama.to
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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