linux-phytec-ti

MD: fix null pointer deference

The md device might not have personality (for example, ddf raid array). The
issue is introduced by 8430e7e0af9a15(md: disconnect device from personality
before trying to remove it)

Reported-by: kernel test robot <xiaolong.ye@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>