erofs: mark z_erofs_lzma_init/erofs_pcpubuf_init w/ __init

They are used during the erofs module init phase. Let's mark it as
__init like any other function.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20230303063731.66760-1-frank.li@vivo.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>