erofs: clean up erofs_iget()

Move inode hash function into inode.c and simplify erofs_iget().

Link: https://lore.kernel.org/r/20230113065226.68801-1-hsiangkao@linux.alibaba.com


Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>