btrfs: make btrfs_do_readpage() to do block-by-block read

btrfs: make btrfs_do_readpage() to do block-by-block read

Currently if btrfs has its block size (the older sector size) smaller
than the page size, btrfs_do_readpage() will handle the range extent by
extent, this is good for performance as it doesn't need to re-lookup the
same extent map again and again.
(Although get_extent_map() already does extra cached em check, thus
the optimization is not that obvious.)

This is totally fine and is a valid optimization, but it has an
assumption that there is no partial uptodate range in the page.

Meanwhile there is an incoming feature, requiring btrfs to skip the full
page read if a buffered write range covers a full block but not a full
page.

In that case, we can have a page that is partially uptodate, and the
current per-extent lookup cannot handle such case.

So here we change btrfs_do_readpage() to do block-by-block read, this
simplifies the following things:

- Remove the need for @iosize variable
  Because we just use sectorsize as our increment.

- Remove @pg_offset, and calculate it inside the loop when needed
  It's just offset_in_folio().

- Use a for() loop instead of a while() loop

This will slightly reduce the read performance for subpage cases, but for
the future where we need to skip already uptodate blocks, it should still
be worth.

For block size == page size, this brings no performance change.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>