[POWERPC] Don't cast a struct pointer to list_head pointer

The casting is safe only when the list_head member is the first member
of the structure, and even then it is better to use the address of the
list_head structure member.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>