erofs-utils: mkfs: fix AUFS whiteouts for tarerofs

Fix the misuse of strcmp() for AUFS whiteout.

Fixes: 73e321a0fb3b ("erofs-utils: lib: consolidate erofs_rebuild_get_dentry()")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250625005307.1225994-1-hsiangkao@linux.alibaba.com