erofs-utils: mkfs: make output stable

erofs-utils: mkfs: make output stable

The iteration order of opendir/readdir depends on filesystem
implementation. Initializng inode->i_ino[0] in the loop causes the
output unstable even though the entries are sorted in
erofs_prepare_dir_file().

In this change,  inode->i_ino[0] is initialized in
erofs_prepare_inode_buffer() instead to make the output stable. (not
affected by readdir())

  # should generate the same output

Test: mkfs.erofs ... inputdir(ext3)
Test: mkfs.erofs ... inputdir(tmpfs)
Signed-off-by: Jooyung Han <jooyung@google.com>
Link: https://lore.kernel.org/r/20241203074531.3728133-1-jooyung@google.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>