md: change the initialization value for a spare device spot to MD_DISK_ROLE_SPARE

The value for spare spot of sb->dev_roles is changed from
MD_DISK_ROLE_FAULTY to MD_DISK_ROLE_SPARE to keep align
with the value when the superblock is firstly created in
userspace.

Signed-off-by: Lidong Zhong <lzhong@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>