Fix atomic_store_voidptr to actually set the value

Why do we have 3 extra copies of the atomic fetch and store for void pointer?

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
Fix initialization of nentry in cache_inode_new_entry

We must initialize a few bits before we have the possibility of
"goto out" with nentry != NULL.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
Keep an atomic pointer to the first export a cache inode belongs to

By keeping a pointer to the first export a cache inode belongs to, we can
fast path check_mapping in the usual case that an inode belongs to only
one export.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>