udf: Use a folio in udf_write_end()

Convert the page to a folio and use the folio APIs.  Replaces three
calls to compound_head() with one.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20240417150416.752929-8-willy@infradead.org>