gve: Check valid ts bit on RX descriptor before hw timestamping

gve: Check valid ts bit on RX descriptor before hw timestamping

The device returns a valid bit in the LSB of the low timestamp byte in
the completion descriptor that the driver should check before
setting the SKB's hardware timestamp. If the timestamp is not valid, do not
hardware timestamp the SKB.

Cc: stable@vger.kernel.org
Fixes: b2c7aeb49056 ("gve: Implement ndo_hwtstamp_get/set for RX timestamping")
Reviewed-by: Joshua Washington <joshwash@google.com>
Signed-off-by: Tim Hostetler <thostet@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20251014004740.2775957-1-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>