btrfs: btrfs_truncate_free_space_cache always allocates path

btrfs_truncate_free_space_cache always allocates a btrfs_path structure
but only uses it when the caller passes a block group.  Let's move the
allocation and free into the conditional.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>