iov_iter: streamline iovec/bvec alignment iteration

iov_iter: streamline iovec/bvec alignment iteration

Rewrite the alignment checking iterators for iovec and bvec to be easier
to read, and also significantly more compact in terms of generated code.
This saves 176 bytes of text on x86-64 for me (with clang-18) and 126
bytes on arm64 (with gcc-13).

In profiles, also saves a bit of time as well for the same workload:

     0.81%     -0.18%  [kernel.vmlinux]  [k] iov_iter_aligned_bvec
     0.48%     -0.09%  [kernel.vmlinux]  [k] iov_iter_is_aligned

which is a nice side benefit as well.

Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>