iomap: Don't get an reference on ZERO_PAGE for direct I/O block zeroing

iomap: Don't get an reference on ZERO_PAGE for direct I/O block zeroing

ZERO_PAGE can't go away, no need to hold an extra reference.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20230315163549.295454-2-dhowells@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>