ovl: use a private non-persistent ino pool

There is no reason to deplete the system's global get_next_ino() pool for
overlay non-persistent inode numbers and there is no reason at all to
allocate non-persistent inode numbers for non-directories.

For non-directories, it is much better to leave i_ino the same as real
i_ino, to be consistent with st_ino/d_ino.

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