ufs: Convert ufs_make_empty() to use a folio

Removes a few hidden calls to compound_head() and uses kmap_local
instead of kmap.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>