fs/affs: make export work with cold dcache

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)

[[email protected]: fix return value check in affs_get_parent()]
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Fabian Frederick <[email protected]>
Signed-off-by: Wei Yongjun <[email protected]>
Suggested-by: Alexander Viro <[email protected]>
Cc: Al Viro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>