client: remove requests from closed MetaSession

If we get a CLOSED message on a session, remove/kick any requests on
that session before tearing it down.  Otherwise, we get a crash like

2013-09-26 03:51:44.704446 7f4d35a46700 10 client.4111 kick_requests for mds.0
2013-09-26 03:51:45.014156 7f4d35a46700 -1 ./include/xlist.h: In function 'xlist<T>::~xlist() [with T = MetaRequest*]' thread 7f4d35a46700 time 2013-09-26 03:51:44.751908
./include/xlist.h: 69: FAILED assert(_size == 0)

 ceph version 0.61.5 (8ee10dc4bb73bdd918873f29c70eedc3c7ef1979)
 1: (MetaSession::~MetaSession()+0x425) [0x4e0105]
 2: (Client::_closed_mds_session(MetaSession*)+0x116) [0x48a696]
 3: (Client::handle_client_session(MClientSession*)+0x2bb) [0x48bf5b]
 4: (Client::ms_dispatch(Message*)+0x56b) [0x4bfa0b]
 5: (DispatchQueue::entry()+0x3f1) [0x621b31]
 6: (DispatchQueue::DispatchThread::entry()+0xd) [0x6191bd]
 7: (()+0x7851) [0x7f4d3c168851]
 8: (clone()+0x6d) [0x7f4d3b09d90d]

Note that this can happen if we fail to reconnect do an MDS during its
reconnect interval.  If that happens, we probably have inodes in our
cache with no caps and things are generally not going to work very well.
This is but one step in improving the situation.

Separate out the two methods since they share little/no behavior.

Signed-off-by: Sage Weil <sage@inktank.com>