iomap,xfs: Convert ->discard_page to ->discard_folio

XFS has the only implementation of ->discard_page today, so convert it
to use folios in the same patch as converting the API.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>