fs: dlm: fix missing unlock on error in accept_from_sock()

Add the missing unlock before return from accept_from_sock()
in the error handling case.

Fixes: 6cde210a ("fs: dlm: add helper for init connection")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David Teigland <teigland@redhat.com>