Make AFS use seq_list_xxx helpers

These proc files show some header before dumping the list, so the
seq_list_start_head() is used.

Signed-off-by: Pavel Emelianov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>