coda: allow removal of busy directories

A directory without children may still be busy when it is the cwd for some
process.  We can safely remove such a directory because the VFS prevents
further operations.  Also we don't need to call d_delete as it is already
called in vfs_rmdir.

Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>