erofs-utils: mount: add support for netlink disconnection

One significant benefit of using the netlink interface is that it
avoids opening NBD device files since openers can hang in the kernel
in some cases (it's hard to resolve with traditional ioctl interfaces,
but I don't want to explain more kernel internals here.)

In fact, using the NBD netlink interface is now always recommended.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20251222074652.1947729-1-hsiangkao@linux.alibaba.com