nbd: Fix memory leak in nbd_add_socket

When adding first socket to nbd, if nsock's allocation failed, the data
structure member "config->socks" was reallocated, but the data structure
member "config->num_connections" was not updated. A memory leak will occur
then because the function "nbd_config_put" will free "config->socks" only
when "config->num_connections" is not zero.

Fixes: 03bf73c3 ("nbd: prevent memory leak")
Reported-by:  <syzbot+934037347002901b8d2a@syzkaller.appspotmail.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>