btrfs: refactor __extent_writepage_io() to do sector-by-sector submission

Unlike the bitmap usage inside raid56, for __extent_writepage_io() we
handle the subpage submission not sector-by-sector, but for each dirty
range we found.

This is not a big deal normally, as the subpage complex code is already
mostly optimized out by the compiler for x86_64.

However for the sake of consistency and for the future of subpage
sector-perfect compression support, this patch does:

- Extract the sector submission code into submit_one_sector()

- Add the needed code to extract the dirty bitmap for subpage case
  There is a small pitfall for non-subpage case, as we cleared page
  dirty before starting writeback, so we have to manually set
  the default dirty_bitmap to 1 for such case.

- Use bitmap_and() to calculate the target sectors we need to submit
  This is done for both subpage and non-subpage cases, and will later
  be expanded to skip inline/compression ranges.

For x86_64, the dirty bitmap will be fixed to 1, with the length of 1,
so we're still doing the same workload per sector.

For larger page sizes, the overhead will be a little larger, as previous
we only need to do one extent_map lookup per-dirty-range, but now it
will be one extent_map lookup per-sector.

But that is the same frequency as x86_64, so we're just aligning the
behavior to x86_64.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>