Unionfs: don't create whiteouts on rightmost branch

Unionfs: don't create whiteouts on rightmost branch

If we are unlinking/rmdir'ing an object on the  rightmost branch, there's no
need to  create a whiteout there: this saves on storage space and inodes.
Also, in the (degenerate) case of having only one branch, this really saves
on whiteouts.

CC: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>