Problem:
- Inside btree `struct node_header::h_tree_type` field is not populated.

Solution:
- Assign tree type to `h_tree_type` variable while initializing a new node.
- Added check in node invariant to ensure this field is populated.
- Fixed one typo error in doc/sm/index.rst file.

Signed-off-by: Kanchan Chaudhari <kanchan.chaudhari@seagate.com>

struct fkvv_head and struct vkvv_head because this order is
expected while performing data recovery.

Signed-off-by: Kanchan Chaudhari <kanchan.chaudhari@seagate.com>