iov_iter: massage iterate_iovec and iterate_kvec to logics similar to iterate_bvec

Premature optimization is the root of all evil...  Trying
to unroll the first pass through the loop makes it harder
to follow and not just for readers - compiler ends up
generating worse code than it would on a "non-optimized"
loop.

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