os/bluestore: fix compiler warnings

os/bluestore: fix compiler warnings

As follows:

/home/jenkins-build/build/workspace/ceph-pull-requests/build/boost/include/boost/intrusive/pointer_plus_bits.hpp: In member function ‘bool BlueStore::ExtentMap::encode_some(uint32_t, uint32_t, ceph::bufferlist&, unsigned int*)’:
/home/jenkins-build/build/workspace/ceph-pull-requests/build/boost/include/boost/intrusive/pointer_plus_bits.hpp:76:7: warning: ‘dummy’ is used uninitialized in this function [-Wuninitialized]
       n = pointer(uintptr_t(p) | (uintptr_t(n) & Mask));
       ^
/home/jenkins-build/build/workspace/ceph-pull-requests/src/os/bluestore/BlueStore.cc:1779:10: note: ‘dummy’ was declared here
   Extent dummy(offset);

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>