erofs-utils: rebuild: only update dev/i_ino[1] pairs for directories

erofs-utils: rebuild: only update dev/i_ino[1] pairs for directories

Since the underlying dev/i_ino[1] pairs are only useful for merged
sub-directories, don't bother with other types of inodes.

Otherwise, the original i_ino[1] could be overwritten unexpectedly,
which impacts resvsp mode at least..

Fixes: f64d9d02576b ("erofs-utils: introduce incremental builds")
Reported-by: Hongzhen Luo <hongzhen@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240702083144.2120808-1-hsiangkao@linux.alibaba.com