block: Use rw_copy_check_uvector()

No need for silly open coding - and struct sg_iovec has exactly the same
layout as struct iovec...

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>