ovl: store real inode pointer in ->i_private

To get from overlay inode to real inode we currently use 'struct
ovl_entry', which has lifetime connected to overlay dentry.  This is okay,
since each overlay dentry had a new overlay inode allocated.

Following patch will break that assumption, so need to leave out ovl_entry.
This patch stores the real inode directly in i_private, with the lowest bit
used to indicate whether the inode is upper or lower.

Lifetime rules remain, using ovl_inode_real() must only be done while
caller holds ref on overlay dentry (and hence on real dentry), or within
RCU protected regions.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>