ima: Rename backing_inode to real_inode

Rename the backing_inode variable to real_inode since it gets its value
from real_inode().

Suggested-by: Amir Goldstein <amir73il@gmail.com>
Co-developed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Acked-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>