crimson/common: make FixedKVNodeLayout::set_val() public

crimson/common: make FixedKVNodeLayout::set_val() public

otherwise clang++ fails with
```
mson/os/seastore/lba_manager/btree/lba_btree_node_impl.cc:183:10: error: 'set_val' is a private member of 'crimson::common::FixedKVNodeLayout<255, unsigned long, ceph_le<unsigned long long>,
crimson::os::seastore::paddr_t, crimson::os::seastore::paddr_le_t, true>::iter_t<false>'
      i->set_val(updated);
         ^
/home/jenkins-build/build/workspace/ceph-perf-crimson/ceph-master/src/crimson/common/fixed_kv_node_layout.h:148:10: note: declared private here
    void set_val(V val) const {
         ^
```

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