NFS: Drop inode after rename

 When doing a rename on top of an existing file that is not in use,
 the inode of the overwritten file will remain in the icache.

 The fix is to decrement i_nlink of the overwritten inode, like we
 do for unlink, rmdir etc already.

 Problem diagnosed by Olaf Kirch. This patch is a slight variation
 on his fix.

 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>