fs/affs: make export work with cold dcache

This adds get_parent function so that nfs client can still work after
cache drop (Tested on NFS v4 with echo 3 > /proc/sys/vm/drop_caches)

[weiyongjun1@huawei.com: fix return value check in affs_get_parent()]
  Link: http://lkml.kernel.org/r/20170123141018.2331-1-weiyj.lk@gmail.com
Link: http://lkml.kernel.org/r/20170109191208.6085-8-fabf@skynet.be


Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Suggested-by: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>