netfs: Remove call to folio_index()

Calling folio_index() is pointless overhead; directly dereferencing
folio->index is fine.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20241005182307.3190401-2-willy@infradead.org


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