So looking at the various callers I'm not sure this API is the
best.  If we want to do something fancy I'd hide the struct iovec
instances entirely with something like:

struct iov_storage {
	struct iovec stack[UIO_FASTIOV], *vec;
}

int iov_iter_import_iovec(struct iov_iter *iter, struct iov_storage *s,
		const struct iovec __user *vec, unsigned long nr_segs,
		int type);

and then add a new helper to free the thing if needed:

void iov_iter_release_iovec(struct iov_storage *s)
{
	if (s->vec != s->stack)
		kfree(s->vec);
}