btrfs: tree-checker: Don't check max block group size as current max chunk size limit is unreliable

[BUG]
A completely valid btrfs will refuse to mount, with error message like:
  BTRFS critical (device sdb2): corrupt leaf: root=2 block=239681536 slot=172 \
    bg_start=12018974720 bg_len=10888413184, invalid block group size, \
    have 10888413184 expect (0, 10737418240]

This has been reported several times as the 4.19 kernel is now being
used. The filesystem refuses to mount, but is otherwise ok and booting
4.18 is a workaround.

Btrfs check returns no error, and all kernels used on this fs is later
than 2011, which should all have the 10G size limit commit.

[CAUSE]
For a 12 devices btrfs, we could allocate a chunk larger than 10G due to
stripe stripe bump up.

__btrfs_alloc_chunk()
|- max_stripe_size = 1G
|- max_chunk_size = 10G
|- data_stripe = 11
|- if (1G * 11 > 10G) {
       stripe_size = 976128930;
       stripe_size = round_up(976128930, SZ_16M) = 989855744

However the final stripe_size (989855744) * 11 = 10888413184, which is
still larger than 10G.

[FIX]
For the comprehensive check, we need to do the full check at chunk read
time, and rely on bg <-> chunk mapping to do the check.

We could just skip the length check for now.

Fixes: fce466ea ("btrfs: tree-checker: Verify block_group_item")
Cc: stable@vger.kernel.org # v4.19+
Reported-by: Wang Yugui <wangyugui@e16-tech.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tree-checker.c

@@ -389,13 +389,11 @@ static int check_block_group_item(struct btrfs_fs_info *fs_info,
 
 	/*
 	 * Here we don't really care about alignment since extent allocator can
-	 * handle it.  We care more about the size, as if one block group is
-	 * larger than maximum size, it's must be some obvious corruption.
+	 * handle it.  We care more about the size.
 	 */
-	if (key->offset > BTRFS_MAX_DATA_CHUNK_SIZE || key->offset == 0) {
+	if (key->offset == 0) {
 		block_group_err(fs_info, leaf, slot,
-			"invalid block group size, have %llu expect (0, %llu]",
-				key->offset, BTRFS_MAX_DATA_CHUNK_SIZE);
+				"invalid block group size 0");
 		return -EUCLEAN;
 	}