librbd: Initialization of class member variables

librbd: Initialization of class member variables

Fixes the coverity issues:

** 1402627 Uninitialized scalar field
CID 1402627 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
2. uninit_member: Non-static class member promotion_state is not
initialized in this constructor nor in any functions that it calls.

** 1402630 Uninitialized scalar field
CID 1402630 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
2. uninit_member: Non-static class member m_promotion_state is not
initialized in this constructor nor in any functions that it calls.

** 1402631 Uninitialized scalar field
CID 1402631 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
2. uninit_member: Non-static class member m_promotion_state is not
initialized in this constructor nor in any functions that it calls.

** 1402632 Uninitialized scalar field
CID 1402632 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
2. uninit_member: Non-static class member tag_tid is not initialized
in this constructor nor in any functions that it calls.

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