btrfs_free_extent() can fail because of ENOMEM. There's no reason to
panic here, we can just abort the transaction.

Fixes: f4b9aa8d ("btrfs_truncate")
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>