btrfs: add structure to keep track of extent range in end_bio_extent_readpage

In end_bio_extent_readpage() we had a strange dance around
extent_start/extent_len.

Hidden behind the strange dance is, it's just calling
endio_readpage_release_extent() on each bvec range.

Here is an example to explain the original work flow:

  Bio is for inode 257, containing 2 pages, for range [1M, 1M+8K)

  end_bio_extent_extent_readpage() entered
  |- extent_start = 0;
  |- extent_end = 0;
  |- bio_for_each_segment_all() {
  |  |- /* Got the 1st bvec */
  |  |- start = SZ_1M;
  |  |- end = SZ_1M + SZ_4K - 1;
  |  |- update = 1;
  |  |- if (extent_len == 0) {
  |  |  |- extent_start = start; /* SZ_1M */
  |  |  |- extent_len = end + 1 - start; /* SZ_1M */
  |  |  }
  |  |
  |  |- /* Got the 2nd bvec */
  |  |- start = SZ_1M + 4K;
  |  |- end = SZ_1M + 4K - 1;
  |  |- update = 1;
  |  |- if (extent_start + extent_len == start) {
  |  |  |- extent_len += end + 1 - start; /* SZ_8K */
  |  |  }
  |  } /* All bio vec iterated */
  |
  |- if (extent_len) {
     |- endio_readpage_release_extent(tree, extent_start, extent_len,
				      update);
	/* extent_start == SZ_1M, extent_len == SZ_8K, uptodate = 1 */

As the above flow shows, the existing code in end_bio_extent_readpage()
is accumulates extent_start/extent_len, and when the contiguous range
stops, calls endio_readpage_release_extent() for the range.

However current behavior has something not really considered:

- The inode can change
  For bio, its pages don't need to have contiguous page_offset.
  This means, even pages from different inodes can be packed into one
  bio.

- bvec cross page boundary
  There is a feature called multi-page bvec, where bvec->bv_len can go
  beyond bvec->bv_page boundary.

- Poor readability

This patch will address the problem:

- Introduce a proper structure, processed_extent, to record processed
  extent range

- Integrate inode/start/end/uptodate check into
  endio_readpage_release_extent()

- Add more comment on each step.
  This should greatly improve the readability, now in
  end_bio_extent_readpage() there are only two
  endio_readpage_release_extent() calls.

- Add inode check for contiguity
  Now we also ensure the inode is the same one before checking if the
  range is contiguous.

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>