Wrapfs: fix NULL ptr deref in ->d_release and ->read_super

Wrapfs: fix NULL ptr deref in ->d_release and ->read_super

It is possible that the dentry private data is NULL in case we ran out of
memory while initializing it in new_dentry_private_data.  So check for NULL
before attempting to release resources.  Similarly, in ->read_super, only
cleanu resources actually needed (the VFS cleans up the rest indirectly).

Signed-off-by: Andrew Burford <aburford@cs.stonybrook.edu>
Signed-off-by: Erez Zadok <ezk@cs.stonybrook.edu>