gianfar: fix jumbo packets+napi+rx overrun crash

      When using jumbo packets and overrunning rx queue with napi enabled,
the following sequence is observed in gfar_add_rx_frag:

   | lstatus                              |       | skb                   |
t  | lstatus,  size, flags                | first | len, data_len, *ptr   |
---+--------------------------------------+-------+-----------------------+
13 | 18002348, 9032, INTERRUPT LAST       | 0     | 9600, 8000,  f554c12e |
12 | 10000640, 1600, INTERRUPT            | 0     | 8000, 6400,  f554c12e |
11 | 10000640, 1600, INTERRUPT            | 0     | 6400, 4800,  f554c12e |
10 | 10000640, 1600, INTERRUPT            | 0     | 4800, 3200,  f554c12e |
09 | 10000640, 1600, INTERRUPT            | 0     | 3200, 1600,  f554c12e |
08 | 14000640, 1600, INTERRUPT FIRST      | 0     | 1600, 0,     f554c12e |
07 | 14000640, 1600, INTERRUPT FIRST      | 1     | 0,    0,     f554c12e |
06 | 1c000080, 128,  INTERRUPT LAST FIRST | 1     | 0,    0,