The ovl_inode contains a copy of lowerpath in lowerstack[0], so the
lowerpath member can be removed.

Use accessor ovl_lowerpath() to get the lowerpath whereever the member
was accessed directly.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>