Currently get_parent (needed to find the exportpoint
above a given dentry) walks the hash table of export points
checking each with is_subdir.  Now it walks up the d_parent
link checking each for membership in the hashtable.

nfsd_lookup currently does that walk too (when crossing
a mountpoint backwards) so the code gets unified.

This approach makes more sense as we move towards a cache
for export information that can be filled on demand.
It also assumes less about the hash table (which will change).