client: do not send release to down mds

client: do not send release to down mds

We can have a session with state where the mds is not up; don't blindly
send a message or we can get

./mds/MDSMap.h: In function 'const entity_inst_t MDSMap::get_inst(int)', in thread '0x7f092aad1910'
./mds/MDSMap.h: 465: FAILED assert(up.count(m))
 ceph version 0.35-6-g6eb8862 (commit:6eb8862e91d142451e256aaa02b34c81a4f21dea)
 1: (ceph::__ceph_assert_fail(char const*, char const*, int, char const*)+0x70) [0x71f11a]
 2: (MDSMap::get_inst(int)+0x4b) [0x6dc191]
 3: (Client::flush_cap_releases()+0x94) [0x677e60]
 4: (Client::tick()+0x1f0) [0x690adc]
 5: (C_C_Tick::finish(int)+0x1c) [0x6f3fbe]
 6: (SafeTimer::timer_thread()+0x2c5) [0x6fbfe5]
 7: (SafeTimerThread::entry()+0x19) [0x6fe399]
 8: (Thread::_entry_func(void*)+0x20) [0x72e944]
 9: /lib/libpthread.so.0 [0x7f092dea573a]
 10: (clone()+0x6d) [0x7f092cba169d]

with a map like

$ ./ceph mds dump 85
2012-01-28 09:37:19.251946 mon <- [mds,dump,85]
2012-01-28 09:37:19.252618 mon.1 -> 'dumped mdsmap epoch 85' (0)
epoch   85
flags   0
created 2012-01-28 09:24:42.411202
modified        2012-01-28 09:28:45.093301
tableserver     0
root    0
session_timeout 60
session_autoclose       300
last_failure    0
last_failure_osd_epoch  18
compat  compat={},rocompat={},incompat={1=base v0.20,2=client writeable ranges,3=default file layouts on dirs,4=dir inode in separate object}
max_mds 1
in      0
up      {}
failed  0
stopped
data_pools      [0]
metadata_pool   1

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