After commit 1c7f49a76773 ("erofs: tidy up EROFS on-disk naming"),
there is a unique `union erofs_inode_i_u` so that we could parse
the union directly.
Besides, it also replaces `inode->i_sb` with `sb` for simplicity.
Signed-off-by: Ferry Meng <mengferry@xxxxxxxxxxxxxxxxx>