btrfs: remove out-of-date comments in btree_writepages()

btrfs: remove out-of-date comments in btree_writepages()

There is a lengthy comment introduced in commit b3ff8f1d380e ("btrfs:
Don't submit any btree write bio if the fs has errors") and commit
c9583ada8cc4 ("btrfs: avoid double clean up when submit_one_bio()
failed"), explaining two things:

- Why we don't want to submit metadata write if the fs has errors

- Why we re-set @ret to 0 if it's positive

However it's no longer uptodate by the following reasons:

- We have better checks nowadays

  Commit 2618849f31e7 ("btrfs: ensure no dirty metadata is written back
  for an fs with errors") has introduced better checks, that if the
  fs is in an error state, metadata writes will not result in any bio
  but instead complete immediately.

  That covers all metadata writes better.

- Mentioned incorrect function name

  The commit c9583ada8cc4 ("btrfs: avoid double clean up when
  submit_one_bio() failed") introduced this ret > 0 handling, but at that
  time the function name submit_extent_page() was already incorrect.

  It was submit_eb_page() that could return >0 at that time,
  and submit_extent_page() could only return 0 or <0 for errors, never >0.

  Later commit b35397d1d325 ("btrfs: convert submit_extent_page() to use
  a folio") changed "submit_extent_page()" to "submit_extent_folio()" in
  the comment, but it doesn't make any difference since the function name
  is wrong from day 1.

  Finally commit 5e121ae687b8 ("btrfs: use buffer xarray for extent
  buffer writeback operations") completely reworked how metadata
  writeback works, and removed submit_eb_page(), leaving only the wrong
  function name in the comment.

  Furthermore the function submit_extent_folio() still exists in the
  latest code base, but is never utilized for metadata writeback, causing
  more confusion.

Just remove the lengthy comment, and replace the "if (ret > 0)" check
with an ASSERT(), since only btrfs_check_meta_write_pointer() can modify
@ret and it returns 0 or <0 for errors.

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