md: remove a spurious call to revalidate_disk_size in update_size

None of the ->resize methods updates the disk size, so calling
revalidate_disk_size here won't do anything.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>