ovl: prepare for lazy lookup of lowerdata inode

Make the code handle the case of numlower > 1 and missing lowerdata
dentry gracefully.

Missing lowerdata dentry is an indication for lazy lookup of lowerdata
and in that case the lowerdata_redirect path is stored in ovl_inode.

Following commits will defer lookup and perform the lazy lookup on
access.

Reviewed-by: Alexander Larsson <alexl@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>