erofs-utils: mount: Check the status of subprocess to avoid infinite loop

In `erofsmount_nbd()`, if the child process forked therein exits
erroneously within `erofsmount_startnbd()`, the NBD device will never
become operational. Since the parent process does not check the child's
status, it will stuck in an infinite loop polling the NBD device state.

This patch ensures the parent process correctly detects this failure and
returns an error accordingly.

Signed-off-by: Yifan Zhao <zhaoyifan28@huawei.com>
Link: https://lore.kernel.org/r/20251212101733.590089-1-zhaoyifan28@huawei.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>