ftruncate an overlayfs inode was checking IS_APPEND() on
overlay inode, but overlay inode does not have the S_APPEND flag.

Check IS_APPEND() on real upper inode instead.
Signed-off-by: NAmir Goldstein <amir73il@gmail.com>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>