iomap: use largest_zero_folio() in iomap_dio_zero()

iomap: use largest_zero_folio() in iomap_dio_zero()

iomap_dio_zero() uses a custom allocated memory of zeroes for padding
zeroes. This was a temporary solution until there was a way to request a
zero folio that was greater than the PAGE_SIZE.

Use largest_zero_folio() function instead of using the custom allocated
memory of zeroes. There is no guarantee from largest_zero_folio()
function that it will always return a PMD sized folio. Adapt the code so
that it can also work if largest_zero_folio() returns a ZERO_PAGE.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>