btrfs: tree-checker: add remap-tree checks to check_block_group_item()

btrfs: tree-checker: add remap-tree checks to check_block_group_item()

Add some write-time checks for block group items relating to the remap
tree.

Here we're checking:

* That the REMAPPED or METADATA_REMAP flags aren't set unless the
  REMAP_TREE incompat flag is also set
* That `remap_bytes` isn't more than the size of the block group
* That `identity_remap_count` isn't more than the number of sectors in
  the block group

Signed-off-by: Mark Harmstone <mark@harmstone.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>