librbd: always initialize local variables

librbd: always initialize local variables

silence warnings like:

src/librbd/io/ImageRequestWQ.cc:576:7: warning: 'length' may be used uninitialized in this function [-Wmaybe-uninitialized]
       if (block_overlapping_io(&m_in_flight_extents, offset, length)) {
       ^~
...
src/librbd/io/ImageRequestWQ.cc:935:12: note: 'length' was declared here
   uint64_t length;
            ^~~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>