iov_iter_bvec_advance(): don't bother with bvec_iter

do what we do for iovec/kvec; that ends up generating better code,
AFAICS.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>