generic: Add rudimetary RENAME_WHITEOUT test

generic: Add rudimetary RENAME_WHITEOUT test

There is no API documentation for RENAME_WHITEOUT. There is no
developer documentation for RENAME_WHITEOUT. There are not comments
in the overlayfs or ext4 implementation of RENAME_WHITEOUT.

Hence, this test simply tries to expose basic RENAME_WHITEOUT
behaviour from ext4 so we can reverse-engineer and verify
bug-for-bug renameat2(RENAME_WHITEOUT) ext4 compatibility.

Note: uses generic/078 just to keep out of the way of the 6-7 other
pending new tests.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>