btrfs: remove redundant initializations for struct btrfs_tree_parent_check

It's pointless to initialize the has_first_key field of the stack local
btrfs_tree_parent_check structure at btrfs_tree_parent_check() and at
btrfs_qgroup_trace_subtree() since all fields not explicitly initialized
are zeroed out. In the case of the first function it's a bit odd because
we are assigning 0 and the field is of type bool, however not incorrect
since a 0 is converted to false.

Just remove the explicit initializations due to their redundancy.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>