btrfs: add definition for EXTENT_TREE_V2

[ Upstream commit 2c7d2a23 ]

This adds the initial definition of the EXTENT_TREE_V2 incompat feature
flag.  This also hides the support behind CONFIG_BTRFS_DEBUG.

THIS IS A IN DEVELOPMENT FORMAT CHANGE, DO NOT USE UNLESS YOU ARE A
DEVELOPER OR A TESTER.

The format is in flux and will be added in stages, any fs will need to
be re-made between updates to the format.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 7081929a ("btrfs: don't abort filesystem when attempting to snapshot deleted subvolume")
Signed-off-by: Sasha Levin <sashal@kernel.org>