[BUG]
If we hit an error from submit_extent_page() inside
__extent_writepage_io(), we could still return 0 to the caller, and
even trigger the warning in btrfs_page_assert_not_dirty().

[CAUSE]
In __extent_writepage_io(), if we hit an error from
submit_extent_page(), we will just clean up the range and continue.

This is completely fine for regular PAGE_SIZE == sectorsize, as we can
only hit one sector in one page, thus after the error we're ensured to
exit and @ret will be saved.

But for subpage case, we may have other dirty subpage range in the page,
and in the next loop, we may succeeded submitting the next range.

In that case, @ret will be overwritten, and we return 0 to the caller,
while we have hit some error.

[FIX]
Introduce @has_error and @saved_ret to record the first error we hit, so
we will never forget what error we hit.

CC: stable@vger.kernel.org # 5.15+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>