btrfs: update the comment for submit_extent_page()

Since commit 390ed29b

 ("btrfs: refactor submit_extent_page() to make
bio and its flag tracing easier"), we are using bio_ctrl structure to
replace some of arguments of submit_extent_page().

But unfortunately that commit didn't update the comment for
submit_extent_page(), thus some arguments are stale like:

- bio_ret
- mirror_num
  Those are all contained in bio_ctrl now.

- prev_bio_flags
  We no longer use this flag to determine if we can merge bios.

Update the comment for submit_extent_page() to keep it up-to-date.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>