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

../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_rwlock_wrlock.S:39
out>) at ./common/RWLock.h:88
r=-110) at osdc/Objecter.cc:1850
out>) at ./include/Context.h:64
common/Timer.cc:268
common/Timer.cc:200
pthread_create.c:308

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