iov_iter: don't require contiguous pages in iov_iter_extract_bvec_pages

iov_iter: don't require contiguous pages in iov_iter_extract_bvec_pages

The iov_iter_extract_pages interface allows to return physically
discontiguous pages, as long as all but the first and last page
in the array are page aligned and page size.  Rewrite
iov_iter_extract_bvec_pages to take advantage of that instead of only
returning ranges of physically contiguous pages.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
[hch: minor cleanups, new commit log]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20241024050021.627350-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>