common: Fix function const, when not all serial() are const.

common: Fix function const, when not all serial() are const.

And then Clang complains:
```
/home/jenkins/workspace/ceph-master/src/common/blkdev.cc:700:13: error: out-of-line definition of 'serial' does not match any declaration in 'Bl
kDev'
int BlkDev::serial(char *serial, size_t max)
            ^~~~~~
/home/jenkins/workspace/ceph-master/src/common/blkdev.h:50:7: note: member declaration does not match because it is const qualified
  int serial(char *serial, size_t max) const;
      ^                                ~~~~~
1 error generated.
```

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>