nfs: fix comments in nfs_rename()

Comments are wrong or out of date.  In particular d_drop() doesn't
free the inode it just unhashes the dentry.  And if target is a
directory then it is not checked for being busy.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>