erofs: simplify erofs_{read,fill}_inode()

 - Switch to on-stack `copied` since it's just 64 bytes;

 - Get rid of `nblks` and derive `i_blocks` directly;

 - Use `inode_set_mtime()` instead of `inode_set_ctime()`
   to follow the ondisk naming;

 - Rearrange the code.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20250310095459.2620647-3-hsiangkao@linux.alibaba.com