src/t_immutable: add ftruncate to append only tests

src/t_immutable: add ftruncate to append only tests

The truncate and ftruncate syscalls check the IS_APPEND() flag
in 2 different vfs code paths and I had to fix both in order to
fix overlayfs IS_APPEND() violations.

Add the ftruncate test to t_immutable, so overlay/030 can verify
the fix.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>