hv_netvsc: Add check for kvmalloc_array

As the potential failure of the kvmalloc_array(),
it should be better to check and restore the 'data'
if fails in order to avoid the dereference of the
NULL pointer.

Fixes: 6ae746711263 ("hv_netvsc: Add per-cpu ethtool stats for netvsc")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Link: https://lore.kernel.org/r/20220314020125.2365084-1-jiasheng@iscas.ac.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
