librbd: Initialize on_finish,locker,force,snap_id

librbd: Initialize on_finish,locker,force,snap_id

** 396154 Uninitialized pointer field
CID 1396154 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
2. uninit_member: Non-static class member on_finish is not initialized
in this constructor nor in any functions that it calls.

** 1396158 Uninitialized pointer field
2. uninit_member: Non-static class member snap_id is not initialized
in this constructor nor in any functions that it calls.
4. uninit_member: Non-static class member force is not initialized
in this constructor nor in any functions that it calls.
CID 1396158 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
6. uninit_member: Non-static class member on_finish is not initialized
in this constructor nor in any functions that it calls.

** 1399593 Uninitialized pointer field
2. uninit_member: Non-static class member locker is not initialized
in this constructor nor in any functions that it calls.
CID 1399593 (#2 of 2): Uninitialized pointer field (UNINIT_CTOR)
4. uninit_member: Non-static class member on_finish is not initialized
in this constructor nor in any functions that it calls.

Signed-off-by: Amit Kumar <amitkuma@redhat.com>