btrfs: deprecate integrity checker feature

The integrity checker feature needs to be enabled at compile time
(BTRFS_FS_CHECK_INTEGRITY) and then enabled by mount options check_int*.

Although it provides some unique features which can not be provided by
any other sanity checks like tree-checker, it does not only have high
CPU and memory overhead, but is also a maintenance burden.

For example, it's the only caller of btrfs_map_block() with
@need_raid_map = 0.

Considering most btrfs developers are not even testing this feature, I'm
here to propose deprecation of this feature.

For now only warning messages will be printed, the feature itself would
still work.

Removal time has been set to 6.7 release.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>