ibmveth: Add multi buffers rx replenishment hcall support

ibmveth: Add multi buffers rx replenishment hcall support

This patch enables batched RX buffer replenishment in ibmveth by
using the new firmware-supported h_add_logical_lan_buffers() hcall
 to submit up to 8 RX buffers in a single call, instead of repeatedly
calling the single-buffer h_add_logical_lan_buffer() hcall.

During the probe, with the patch, the driver queries ILLAN attributes
to detect IBMVETH_ILLAN_RX_MULTI_BUFF_SUPPORT bit. If the attribute is
present, rx_buffers_per_hcall is set to 8, enabling batched replenishment.
Otherwise, it defaults to 1, preserving the original upstream behavior
 with no change in code flow for unsupported systems.

The core rx replenish logic remains the same. But when batching
is enabled, the driver aggregates up to 8 fully prepared descriptors
into a single h_add_logical_lan_buffers() hypercall. If any allocation
or DMA mapping fails while preparing a batch, only the successfully
prepared buffers are submitted, and the remaining are deferred for
the next replenish cycle.

If at runtime the firmware stops accepting the batched hcall—e,g,
after a Live Partition Migration (LPM) to a host that does not
support h_add_logical_lan_buffers(), the hypercall returns H_FUNCTION.
In that case, the driver transparently disables batching, resets
rx_buffers_per_hcall to 1, and falls back to the single-buffer hcall
in next future replenishments to take care of these and future buffers.

Test were done on systems with firmware that both supports and
does not support the new h_add_logical_lan_buffers hcall.

On supported firmware, this reduces hypercall overhead significantly
over multiple buffers. SAR measurements showed about a 15% improvement
in packet processing rate under moderate RX load, with heavier traffic
seeing gains more than 30%

Signed-off-by: Mingming Cao <mmc@linux.ibm.com>
Reviewed-by: Brian King <bjking1@linux.ibm.com>
Reviewed-by: Haren Myneni <haren@linux.ibm.com>
Reviewed-by: Dave Marquardt <davemarq@linux.ibm.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250719091356.57252-1-mmc@linux.ibm.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>