ncpfs: fix rename over directory with dangling references

ncpfs does not handle references to unlinked directories (or so it would
seem given the ncp_rmdir check).  Since it is also possible to rename over
an empty directory, perform the same check here.

CC: Petr Vandrovec <petr@vandrovec.name>
CC: linux-kernel@vger.kernel.org
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>