erofs-utils: lib: avoid trailing '\n' in erofs_nbd_get_identifier()

The trailing '\n' shouldn't be part of `backend`.

Fixes: 5d3efc9babf3 ("erofs-utils: mount: enable autoclear for NBD devices")
Reported-and-tested-by: Chengyu Zhu <hudsonzhu@tencent.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250905033955.1351125-1-hsiangkao@linux.alibaba.com
2 files changed