iov_iter: unify iterate_iovec and iterate_kvec

The differences between iterate_iovec and iterate_kvec are minor:
	* kvec callback is treated as if it returned 0
	* initialization of __p is with i->iov and i->kvec resp.
which is trivially dealt with.

No code generation changes - compiler is quite capable of turning
	left = ((void)(STEP), 0);
	__v.iov_len -= left;
(with no accesses to left downstream) and
	(void)(STEP);
into the same code.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>