udf: Convert udf_page_mkwrite() to use a folio

Convert the vm_fault page to a folio, then use it throughout.
Replaces five 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: <20240417150416.752929-7-willy@infradead.org>