gianfar: prevent integer wrapping in the rx handler

gianfar: prevent integer wrapping in the rx handler

When the frame check sequence (FCS) is split across the last two frames
of a fragmented packet, part of the FCS gets counted twice, once when
subtracting the FCS, and again when subtracting the previously received
data.

For example, if 1602 bytes are received, and the first fragment contains
the first 1600 bytes (including the first two bytes of the FCS), and the
second fragment contains the last two bytes of the FCS:

  'skb->len == 1600' from the first fragment

  size  = lstatus & BD_LENGTH_MASK; # 1602
  size -= ETH_FCS_LEN;              # 1598
  size -= skb->len;                 # -2

Since the size is unsigned, it wraps around and causes a BUG later in
the packet handling, as shown below:

  kernel BUG at ./include/linux/skbuff.h:2068!
  Oops: Exception in kernel mode, sig: 5 [#1]
  ...
  NIP [c021ec60] skb_pull+0x24/0x44
  LR [c01e2fbc] gfar_clean_rx_ring+0x498/0x690
  Call Trace:
  [df7edeb0] [c01e2c1c] gfar_clean_rx_ring+0xf8/0x690 (unreliable)
  [df7edf20] [c01e33a8] gfar_poll_rx_sq+0x3c/0x9c
  [df7edf40] [c023352c] net_rx_action+0x21c/0x274
  [df7edf90] [c0329000] __do_softirq+0xd8/0x240
  [df7edff0] [c000c108] call_do_irq+0x24/0x3c
  [c0597e90] [c00041dc] do_IRQ+0x64/0xc4
  [c0597eb0] [c000d920] ret_from_except+0x0/0x18
  --- interrupt: 501 at arch_cpu_idle+0x24/0x5c

Change the size to a signed integer and then trim off any part of the
FCS that was received prior to the last fragment.

Fixes: 6c389fc931bc ("gianfar: fix size of scatter-gathered frames")
Signed-off-by: Andy Spencer <aspencer@spacex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>