dpaa_ethernet: fix kmemleak false-positive reports for skb

Kmemleak reports following false positive memory leaks for each sk
buffers allocated by DPAA (build_skb()) in _dpa_bp_add_8_bufs:

unreferenced object 0xffff00000310d100 (size 224):
  comm "swapper/0", pid 1, jiffies 4294892973 (age 64.948s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<000000003c47dcde>] kmem_cache_alloc+0x1e0/0x4e4
    [<0000000008f09091>] __build_skb+0x38/0xd4
    [<000000001358eefb>] build_skb+0x28/0x9c
    [<000000005213aa43>] dpaa_eth_refill_bpools+0xe0/0x35c
    [<00000000d936eefb>] dpaa_eth_priv_probe+0x758/0x794
    [<00000000f1140028>] platform_drv_probe+0x60/0xb4
    [<000000002962c77a>] really_probe+0xdc/0x4a0
    [<0000000044f4dd2b>] driver_probe_device+0x64/0xcc
    [<00000000ae32fd5d>] device_driver_attach+0xcc/0xd4
    [<00000000163633a3>] __driver_attach+0x90/0x130
    [<0000000031f611a6>] bus_for_each_dev+0x7c/0xd0
    [<000000007095e9c3>] driver_attach+0x30/0x3c
    [<0000000035c9989f>] bus_add_driver+0x114/0x200
    [<00000000b14c744e>] driver_register+0x84/0x140
    [<00000000d133e73f>] __platform_driver_register+0x54/0x60
    [<000000006e2c296f>] dpa_load+0x6c/0xb0

This happens because DPAA allocates SK buffers and then passes pointers
for DMA usages, which belongs to DEV MMIO space.  Kmemleak does not scan
IO memory and so reports memory leaks.

Hence, mark allocated sk buffers as false positive explicitly.

Signed-off-by: Zumeng Chen <zumeng.chen@windriver.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>
Signed-off-by: Meng Li <Meng.Li@windriver.com>