igc: skip RX timestamp header for frame preemption verification

When RX hardware timestamping is enabled, a 16-byte inline timestamp header
is added to the start of the packet buffer, causing FPE handshake
verification to fail.

Because an incorrect packet buffer is passed to igc_fpe_handle_mpacket(),
the mem_is_zero() check inspects the timestamp metadata instead of the
actual mPacket payload. As a result, valid Verify/Response mPackets can be
missed when inline RX timestamps are present.

Pass pktbuf + pkt_offset to igc_fpe_handle_mpacket() so it inspects the
actual mPacket payload instead of the timestamp header.

Fixes: 5422570c0010 ("igc: add support for frame preemption verification")
Co-developed-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
Signed-off-by: KhaiWenTan <khai.wen.tan@linux.intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>