erofs: get rid of the leftover PAGE_SIZE in dir.c

Convert the last hardcoded PAGE_SIZEs of uncompressed cases.

Reviewed-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220619150940.121005-1-hsiangkao@linux.alibaba.com