nfs: Use a folio in nfs_get_link()

Mirror the changes to __page_get_link() by retrieving a folio from
the page cache instead of a page.  Removes two hidden calls to
compound_head().

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/20250514171316.3002934-3-willy@infradead.org


Signed-off-by: Christian Brauner <brauner@kernel.org>