befs: make export work with cold dcache

based on commit b3b42c0d
("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)

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>