net: mana: Fall back to standard MTU when PF reports adapter_mtu of 0

net: mana: Fall back to standard MTU when PF reports adapter_mtu of 0

Commit d7709812e13d ("net: mana: hardening: Validate adapter_mtu from
MANA_QUERY_DEV_CONFIG") rejected any adapter_mtu value smaller than
ETH_MIN_MTU + ETH_HLEN, including 0, returning -EPROTO and failing
mana_probe().

Some older PF firmware versions still in the field report
adapter_mtu as 0 in the MANA_QUERY_DEV_CONFIG response. With the
hardening check in place, the MANA VF driver now fails to load on
those hosts, breaking networking entirely for guests.

MANA hardware always supports the standard Ethernet MTU. Treat a
reported adapter_mtu of 0 as "the PF did not advertise a value" and
fall back to ETH_FRAME_LEN, the same value used for the pre-V2
message version path. Only jumbo frames remain unavailable until
the PF reports a valid MTU.

Other small-but-nonzero bogus values are still rejected, preserving
the original protection against the unsigned-subtraction wrap that
would otherwise let ndev->max_mtu underflow to a huge value.

Fixes: d7709812e13d ("net: mana: hardening: Validate adapter_mtu from MANA_QUERY_DEV_CONFIG")
Signed-off-by: Erni Sri Satya Vennela <ernis@linux.microsoft.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260619055348.467224-1-ernis@linux.microsoft.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>