erofs: allow readdir() to be interrupted

In a quick slow device, readdir() may loop for long time in large
directory, let's give a chance to allow it to be interrupted by
userspace.

Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250710073619.4083422-1-chao@kernel.org


[ Gao Xiang: move cond_resched() to the end of the while loop. ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>