osdc: use operator=(...) instead of claim()

osdc: use operator=(...) instead of claim()

for silencing the warning like:
```
../src/osdc/ObjectCacher.cc: In member function ‘ObjectCacher::BufferHead* ObjectCacher::Object::split(ObjectCacher::BufferHead*, loff_t)’:
../src/osdc/ObjectCacher.cc:138:20: warning: ‘void ceph::buffer::v15_2_0::list::claim(ceph::buffer::v15_2_0::list&)’ is deprecated: in favor of operator=(list&&) [-Wdeprecated-declarations]
  138 |   bl.claim(left->bl);
      |                    ^
```

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