btrfs: prevent inline data extents read from touching blocks beyond its range

btrfs: prevent inline data extents read from touching blocks beyond its range

Currently reading an inline data extent will zero out the remaining
range in the page.

This is not yet causing problems even for block size < page size
(subpage) cases because:

1) An inline data extent always starts at file offset 0
   Meaning at page read, we always read the inline extent first, before
   any other blocks in the page. Then later blocks are properly read out
   and re-fill the zeroed out ranges.

2) Currently btrfs will read out the whole page if a buffered write is
   not page aligned
   So a page is either fully uptodate at buffered write time (covers the
   whole page), or we will read out the whole page first.
   Meaning there is nothing to lose for such an inline extent read.

But it's still not ideal:

- We're zeroing out the page twice
  Once done by read_inline_extent()/uncompress_inline(), once done by
  btrfs_do_readpage() for ranges beyond i_size.

- We're touching blocks that don't belong to the inline extent
  In the incoming patches, we can have a partial uptodate folio, of
  which some dirty blocks can exist while the page is not fully uptodate:

  The page size is 16K and block size is 4K:

  0         4K        8K        12K        16K
  |         |         |/////////|          |

  And range [8K, 12K) is dirtied by a buffered write, the remaining
  blocks are not uptodate.

  If range [0, 4K) contains an inline data extent, and we try to read
  the whole page, the current behavior will overwrite range [8K, 12K)
  with zero and cause data loss.

So to make the behavior more consistent and in preparation for future
changes, limit the inline data extents read to only zero out the range
inside the first block, not the whole page.

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