iov_iter: fix iov_iter_type

Commit 875f1d07 ("iov_iter: add ITER_BVEC_FLAG_NO_REF flag")
introduces one extra flag of ITER_BVEC_FLAG_NO_REF, and this flag
is stored into iter->type.

However, iov_iter_type() doesn't consider the new added flag, fix
it by masking this flag in iov_iter_type().

Fixes: 875f1d07

 ("iov_iter: add ITER_BVEC_FLAG_NO_REF flag")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>