osdc/Objecter: cancel timeout before clearing op->session

osdc/Objecter: cancel timeout before clearing op->session

The C_CancelOp path assumes op->session != NULL.  Cancel that op before
we clear it.  This fixes a crash like

#0  pthread_rwlock_wrlock () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_rwlock_wrlock.S:39
#1  0x00007fc82690a4b1 in RWLock::get_write (this=0x18, lockdep=<optimized out>) at ./common/RWLock.h:88
#2  0x00007fc8268f4d79 in Objecter::op_cancel (this=0x1f61830, s=0x0, tid=0, r=-110) at osdc/Objecter.cc:1850
#3  0x00007fc8268ba449 in Context::complete (this=0x1f68c20, r=<optimized out>) at ./include/Context.h:64
#4  0x00007fc8269769aa in RWTimer::timer_thread (this=0x1f61950) at common/Timer.cc:268
#5  0x00007fc82697a85d in RWTimerThread::entry (this=<optimized out>) at common/Timer.cc:200
#6  0x00007fc82651ce9a in start_thread (arg=0x7fc7e3fff700) at pthread_create.c:308

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