SUNRPC: rpc_unlink() must check for unhashed dentries

A prior call to rpc_depopulate() by rpc_rmdir() on the parent directory may
have already called simple_unlink() on this entry.
Add the same check to rpc_rmdir(). Also remove a redundant call to
rpc_close_pipes() in rpc_rmdir.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
(cherry picked from 0bbfb9d20f6437c4031aa3bf9b4d311a053e58e3 commit)