9P: symlink: put the symlink ref after creation, because the client doesn't actually get a ref (like mkdir)

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
FSAL_VFS: fixes in makesymlink and makedir

makedir:
 - Do not try to unlinkat the directory we just created on success
   (it currently works because unlinkat fails with EISDIR)
 - close dir_fd on success, as it currently is done (and as it is
   done in other functions)

makesymlink:
 - fsal_restore_ganesha_credentials was called twice for some reason?
 - close dir_fd on success

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>