Unionfs: create new symlinks only in first branch

Unionfs: create new symlinks only in first branch

When creating a new symlink, always create it in the first branch, which is
always writeable, not in the branch which may have a whiteout in it.  This
makes the policy for the creation of new symlinks consistent with that of
new files/directories, as well as improves efficiency a bit.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>