erofs: simplify z_erofs_pcluster_readmore()

Get rid of unnecessary label `skip'. No logic changes.

Link: https://lore.kernel.org/r/20220529055425.226363-4-xiang@kernel.org
Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>