ovl: store upper real file in ovl_file struct

ovl: store upper real file in ovl_file struct

When an overlayfs file is opened as lower and then the file is copied up,
every operation on the overlayfs open file will open a temporary backing
file to the upper dentry and close it at the end of the operation.

Store the upper real file along side the original (lower) real file in
ovl_file instead of opening a temporary upper file on every operation.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>