btrfs: add unlikely to unexpected error case in extent_writepages()

btrfs: add unlikely to unexpected error case in extent_writepages()

We don't expect to hit errors and log the error message, so add the
unlikely annotation to make it clear and to hint the compiler that it may
reorganize code to be more efficient.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <asj@kernel.org>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>