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>

fs/btrfs/extent-tree.c

@@ -4797,7 +4797,7 @@ btrfs_init_new_buffer(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 	    !test_bit(BTRFS_ROOT_RESET_LOCKDEP_CLASS, &root->state))
 		lockdep_owner = BTRFS_FS_TREE_OBJECTID;
 
-	/* btrfs_clean_tree_block() accesses generation field. */
+	/* btrfs_clear_buffer_dirty() accesses generation field. */
 	btrfs_set_header_generation(buf, trans->transid);
 
 	/*