net: hns: fix ethtool_get_strings overflow in hns driver

hns_get_sset_count() returns HNS_NET_STATS_CNT and the data space allocated
is not enough for ethtool_get_strings(), which will cause random memory
corruption.

When SLAB and DEBUG_SLAB are both enabled, memory corruptions like the
the following can be observed without this patch:
[   43.115200] Slab corruption (Not tainted): Acpi-ParseExt start=ffff801fb0b69030, len=80
[   43.115206] Redzone: 0x9f911029d006462/0x5f78745f31657070.
[   43.115208] Last user: [<5f7272655f746b70>](0x5f7272655f746b70)
[   43.115214] 010: 70 70 65 31 5f 74 78 5f 70 6b 74 00 6b 6b 6b 6b  ppe1_tx_pkt.kkkk
[   43.115217] 030: 70 70 65 31 5f 74 78 5f 70 6b 74 5f 6f 6b 00 6b  ppe1_tx_pkt_ok.k
[   43.115218] Next obj: start=ffff801fb0b69098, len=80
[   43.115220] Redzone: 0x706d655f6f666966/0x9f911029d74e35b.
[   43.115229] Last user: [<ffff0000084b11b0>](acpi_os_release_object+0x28/0x38)
[   43.115231] 000: 74 79 00 6b 6b 6b 6b 6b 70 70 65 31 5f 74 78 5f  ty.kkkkkppe1_tx_
[   43.115232] 010: 70 6b 74 5f 65 72 72 5f 63 73 75 6d 5f 66 61 69  pkt_err_csum_fai

Signed-off-by: Timmy Li <lixiaoping3@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
