net: hns3: Fix warning bug when doing lp selftest

The napi_alloc_skb is excepted to be called under the
non-preemptible code path when it is called by hns3_clean_rx_ring
during loopback selftest, otherwise the below warning will be
logged:

[   92.420780] BUG: using smp_processor_id() in preemptible
[00000000] code: ethtool/1873
<SNIP>
[   92.463202]  check_preemption_disabled+0xf8/0x100
[   92.467893]  debug_smp_processor_id+0x1c/0x28
[   92.472239]  __napi_alloc_skb+0x30/0x130
[   92.476158]  hns3_clean_rx_ring+0x118/0x5f0 [hns3]
[   92.480941]  hns3_self_test+0x32c/0x4d0 [hns3]
[   92.485375]  ethtool_self_test+0xdc/0x1e8
[   92.489372]  dev_ethtool+0x1020/0x1da8
[   92.493109]  dev_ioctl+0x188/0x3a0
[   92.496499]  sock_do_ioctl+0xf4/0x208
[   92.500148]  sock_ioctl+0x228/0x3e8
[   92.503626]  do_vfs_ioctl+0xc4/0x880
[   92.507189]  SyS_ioctl+0x94/0xa8
[   92.510404]  el0_svc_naked+0x30/0x34

This patch fix it by disabling preemption when calling
hns3_clean_rx_ring during loopback selftest.

Fixes: c39c4d98 ("net: hns3: Add mac loopback selftest support in hns3 driver")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>