btrfs: tree-checker: introduce checks for FREE_SPACE_INFO

Introduce checks for FREE_SPACE_INFO item, which include:

- Key alignment check
  The objectid is the logical bytenr of the chunk/bg, and offset is the
  length of the chunk/bg, thus they should all be aligned to the fs
  block size.

- Item size check
  The FREE_SPACE_INFO should a fix size.

- Flags check
  The flags member should have no other flags than
  BTRFS_FREE_SPACE_USING_BITMAPS.

  For future expansion, introduce a new macro
  BTRFS_FREE_SPACE_FLAGS_MASK for such checks.

  And since we're here, the BTRFS_FREE_SPACE_USING_BITMAPS should not
  use unsigned long long, as the flags is only 32 bits wide.
  So fix that to use unsigned long.

- Extent count check
  That member shows how many free space bitmap/extent items there are
  inside the chunk/bg.

  We know the chunk size (from key->offset), thus there should be at
  most (key->offset >> sectorsize_bits) blocks inside the chunk.
  Use that value as the upper limit and if that counter is larger than
  that, there is a high chance it's a bitflip in high bits.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

include/uapi/linux/btrfs_tree.h

@@ -1245,7 +1245,8 @@ struct btrfs_free_space_info {
 	__le32 flags;
 } __attribute__ ((__packed__));
 
-#define BTRFS_FREE_SPACE_USING_BITMAPS (1ULL << 0)
+#define BTRFS_FREE_SPACE_USING_BITMAPS	(1UL << 0)
+#define BTRFS_FREE_SPACE_FLAGS_MASK	(BTRFS_FREE_SPACE_USING_BITMAPS)
 
 #define BTRFS_QGROUP_LEVEL_SHIFT		48
 static inline __u16 btrfs_qgroup_level(__u64 qgroupid)