mds: fix Resetter locking

ceph-mds --reset-journal didn't work; it would deadlock waiting for
the osdmap.  Comparing the init code in the Dumper (that worked) with
that in the Resetter (that didn't), I noticed the lock had to be
released before waiting for the osdmap.

Now the resetter works.  However, both the resetter and the dumper
fail an assertion after they've performed their task; I didn't look
into it:

../../src/msg/SimpleMessenger.cc: In function 'void SimpleMessenger::reaper()' t
hread 7fdc188d27c0 time 2013-12-19 04:48:16.930895
../../src/msg/SimpleMessenger.cc: 230: FAILED assert(!cleared)
 ceph version 0.72.1-6-g6bca44e (6bca44ec129d11f1c4f38357db8ae435616f2c7c)
 1: (SimpleMessenger::reaper()+0x706) [0x880da6]
 2: (SimpleMessenger::wait()+0x36f) [0x88180f]
 3: (Resetter::reset()+0x714) [0x56e664]
 4: (main()+0x1359) [0x562769]
 5: (__libc_start_main()+0xf5) [0x3632e21b45]
 6: /l/tmp/build/ceph/build/src/ceph-mds() [0x564e49]
 NOTE: a copy of the executable, or `objdump -rdS <executable>` is needed to int
erpret this.
2013-12-19 04:48:16.934093 7fdc188d27c0 -1 ../../src/msg/SimpleMessenger.cc: In
function 'void SimpleMessenger::reaper()' thread 7fdc188d27c0 time 2013-12-19 04
:48:16.930895
../../src/msg/SimpleMessenger.cc: 230: FAILED assert(!cleared)

Signed-off-by: Alexandre Oliva <oliva@gnu.org>