udf: Convert udf_symlink_getattr() to use a folio

We're getting this from the page cache, so it's definitely a folio.
Saves a call to compound_head() hidden in put_page().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20240417150416.752929-6-willy@infradead.org>