Wrapfs: fix NULL pointer dereference when stacking wrapfs on top of itself.

Wrapfs: fix NULL pointer dereference when stacking wrapfs on top of itself.

Wrapfs assumes that the d_fsdata field has already been initialized whenever
it is passed one of its own dentries, but it doesn't give the lower fs a chance
to allocate their d_fsdata field when it creates a new lower_dentry. This is
fixed by simply replacing a call to d_add in __wrapfs_lookup with a call to
->lookup on the lower fs inode.

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