_scif_init() free scif_dev in the free_sdev erro path,
but _scif_exit will free it again when module exit, it
cause BUG_ON issue,

  kernel BUG at mm/slub.c:3944!
  invalid opcode: 0000 [#1] SMP KASAN PTI

Set scif_dev to NULL in scif_destroy_scifdev() to fix it.

Cc: Sudeep Dutt <sudeep.dutt@intel.com>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>