[PROBLEM]
There are some reports of corrupted fs which can't be mounted due to
corrupted extent tree.

However under such situation, it's more likely the fs/subvolume trees
are still fine.

For such case we normally go btrfs-restore and salvage as much as we
can. However btrfs-restore can't list subvolumes as "btrfs subv list",
making it harder to restore a fs.

[ENHANCEMENT]
This patch will introduce a new mount option "rescue=skipbg" to skip
the mount time block group scan, and use chunk info solely to populate
fake block group cache.

The mount option has the following dependency:
- RO mount
  Obviously.

- No dirty log.
  Either there is no log, or use rescue=nologreplay mount option.

- No way to remoutn RW
  Similar to rescue=nologreplay option.

This allow kernel to accept all extent tree corruption, even when the
whole extent tree is corrupted, and allow user to salvage data and
subvolume info.

Signed-off-by: Qu Wenruo <wqu@suse.com>