tools/ceph_dedup: Add const qualifiers and reference parameters

tools/ceph_dedup: Add const qualifiers and reference parameters

Improve code quality in ceph_dedup tool by:

- Adding const qualifiers to member functions and parameters where appropriate
- Converting parameter passing to use references instead of value copies
  for complex objects

These changes enhance code readability, better express intent through
const-correctness, and improve performance by avoiding unnecessary deep
copies.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>