btrfs: fix comment referring to no longer existing btrfs_clean_tree_block()

There's a comment at btrfs_init_new_buffer() that refers to a function
named btrfs_clean_tree_block(), however the function was renamed to
btrfs_clear_buffer_dirty() in commit 190a8339 ("btrfs: rename
btrfs_clean_tree_block to btrfs_clear_buffer_dirty"). So update the
comment to refer to the current name.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>