ovl: deduplicate lowerdata and lowerstack[]

The ovl_inode contains a copy of lowerdata in lowerstack[], so the
lowerdata inode member can be removed.

Use accessors ovl_lowerdata*() to get the lowerdata whereever the member
was accessed directly.

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