ovl: fix regression in fsnotify of overlay merge dir

A re-factoring patch in NFS export series has passed the wrong argument
to ovl_get_inode() causing a regression in the very recent fix to
fsnotify of overlay merge dir.

The regression has caused merge directory inodes to be hashed by upper
instead of lower real inode, when NFS export and directory indexing is
disabled. That caused an inotify watch to become obsolete after directory
copy up and drop caches.

LTP test inotify07 was improved to catch this regression.
The regression also caused multiple redirect dirs to same origin not to
be detected on lookup with NFS export disabled. An xfstest was added to
cover this case.

Fixes: 0aceb53e ("ovl: do not pass overlay dentry to ovl_get_inode()")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>