bps: refactor the debug check functions

The checkpatch does not permit to modify several parts of inner debug
check functions complaining about too big indentation. The modification
will be required further to implement the LogN offset in the BPS tree,
so this patch refactors the functions and introduces a helper function
for this: bps_tree_debug_insert_and_move_next.

The refactored functions are:
- bps_tree_debug_check_insert_and_move_to_right_inner
- bps_tree_debug_check_insert_and_move_to_left_inner
- bps_tree_debug_check_insert_and_move_to_right_leaf
- bps_tree_debug_check_insert_and_move_to_left_leaf

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring